Makefile: allow user CFLAGS to override out warning flags

The cc-option filters out unknown flags which make the compiler error
out.
Note that Clang enables Wunused-command-line-argument by default and
merely emits a warning but incurs no error, when it encounters such a
flag.  GCC on the other hand exits unsuccessfully when it is passed a
flag of the form -Wunknown-warning, but no diagnostic is produced for
-Wno-unknown-warning unless other diagnostics are produced.

LICENSE

@@ -338,3 +338,26 @@ consider it more useful to permit linking proprietary applications with the
 library.  If this is what you want to do, use the GNU Lesser General
 Public License instead of this License.
 
+---
+
+Copyright 2019 the fsverity-utils authors
+
+Permission is hereby granted, free of charge, to any person
+obtaining a copy of this software and associated documentation files
+(the "Software"), to deal in the Software without restriction,
+including without limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of the Software,
+and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

Makefile

@@ -1,9 +1,20 @@
 # Makefile to build x48ng without autotools
+#
+# The cc-option function and the C{,PP}FLAGS logic were copied from the
+# fsverity-utils project.
+# https://git.kernel.org/pub/scm/fs/fsverity/fsverity-utils.git/
+# The governing license can be found in the LICENSE file or at
+# https://opensource.org/license/MIT.
 
 PREFIX = /usr
 DOCDIR = $(PREFIX)/doc/x48ng
 MANDIR = $(PREFIX)/man
 
+CFLAGS ?= -g -O2
+FULL_WARNINGS = no
+WITH_X11 ?= yes
+WITH_SDL ?= yes
+
 VERSION_MAJOR = 0
 VERSION_MINOR = 37
 PATCHLEVEL = 99
@@ -24,82 +35,66 @@ DOTOS = src/emu_serial.o \
 
 MAKEFLAGS +=-j$(NUM_CORES) -l$(NUM_CORES)
 
-WITH_X11 ?= yes
+cc-option = $(shell if $(CC) $(1) -c -x c /dev/null -o /dev/null > /dev/null 2>&1; \
-WITH_SDL ?= yes
+	      then echo $(1); fi)
 
-OPTIM ?= 2
+ifeq ($(FULL_WARNINGS), no)
+EXTRA_WARNING_FLAGS := -Wno-unused-function \
+	-Wno-redundant-decls \
+	$(call cc-option,-Wno-maybe-uninitialized) \
+	$(call cc-option,-Wno-discarded-qualifiers) \
+	$(call cc-option,-Wno-uninitialized) \
+	$(call cc-option,-Wno-ignored-qualifiers)
+endif
+
+ifeq ($(FULL_WARNINGS), yes)
+EXTRA_WARNING_FLAGS := -Wunused-function \
+	-Wredundant-decls \
+	-fsanitize-trap \
+	$(call cc-option,-Wunused-variable)
+endif
+
+override CFLAGS := -std=c11 \
+	-I./src/ -D_GNU_SOURCE=1 \
+	-DVERSION_MAJOR=$(VERSION_MAJOR) \
+	-DVERSION_MINOR=$(VERSION_MINOR) \
+	-DPATCHLEVEL=$(PATCHLEVEL) \
+	-Wall -Wextra -Wpedantic \
+	-Wformat=2 -Wshadow \
+	-Wwrite-strings -Wstrict-prototypes -Wold-style-definition \
+	-Wnested-externs -Wmissing-include-dirs \
+	-Wdouble-promotion \
+	-Wno-sign-conversion \
+	-Wno-unused-variable \
+	-Wno-unused-parameter \
+	-Wno-conversion \
+	-Wno-format-nonliteral \
+	$(call cc-option,-Wjump-misses-init) \
+	$(call cc-option,-Wlogical-op) \
+	$(call cc-option,-Wno-unknown-warning-option) \
+	$(EXTRA_WARNING_FLAGS) \
+	$(CFLAGS)
 
-CFLAGS += -std=c11 -g -O$(OPTIM) -I./src/ -D_GNU_SOURCE=1 -DVERSION_MAJOR=$(VERSION_MAJOR) -DVERSION_MINOR=$(VERSION_MINOR) -DPATCHLEVEL=$(PATCHLEVEL)
 LIBS = -lm
 
 ### lua
-CFLAGS += $(shell pkg-config --cflags lua)
+override CFLAGS += $(shell pkg-config --cflags lua)
 LIBS += $(shell pkg-config --libs lua)
 
 ### debugger
-CFLAGS += $(shell pkg-config --cflags readline)
+override CFLAGS += $(shell pkg-config --cflags readline)
 LIBS += $(shell pkg-config --libs readline)
 
 ### Text UI
-CFLAGS += $(shell pkg-config --cflags ncursesw) -DNCURSES_WIDECHAR=1
+override CFLAGS += $(shell pkg-config --cflags ncursesw) -DNCURSES_WIDECHAR=1
 LIBS += $(shell pkg-config --libs ncursesw)
 
-# Warnings
-FULL_WARNINGS = no
-
-# Useful warnings
-CFLAGS += -Wall -Wextra -Wpedantic \
-	  -Wformat=2 -Wshadow \
-	  -Wwrite-strings -Wstrict-prototypes -Wold-style-definition \
-	  -Wnested-externs -Wmissing-include-dirs \
-	  -Wdouble-promotion
-# GCC warnings that Clang doesn't provide:
-ifeq ($(CC),gcc)
-	CFLAGS += -Wjump-misses-init -Wlogical-op
-endif
-ifeq ($(CC),clang)
-	CFLAGS += -Wno-unknown-warning-option
-endif
-
-# Ok we still disable some warnings for (hopefully) good reasons
-# Not useful warnings
-CFLAGS += -Wno-sign-conversion
-CFLAGS += -Wno-unused-variable
-CFLAGS += -Wno-unused-parameter
-CFLAGS += -Wno-conversion
-# 1. The debugger uses Xprintf format strings declared as char*, triggering this warning
-CFLAGS += -Wno-format-nonliteral
-
-ifeq ($(FULL_WARNINGS), no)
-	CFLAGS += -Wno-unused-function
-	CFLAGS += -Wno-redundant-decls
-	ifeq ($(CC),gcc)
-		CFLAGS += -Wno-maybe-uninitialized
-		CFLAGS += -Wno-discarded-qualifiers
-	endif
-	ifeq ($(CC),clang)
-		CFLAGS += -Wno-uninitialized
-		CFLAGS += -Wno-ignored-qualifiers
-	endif
-else
-	# CFLAGS += -Wunused-variable
-	# CFLAGS += -Wunused-parameter
-	CFLAGS += -Wunused-function
-	CFLAGS += -Wredundant-decls
-	# CFLAGS += -Wconversion
-	# CFLAGS += -fsanitize=undefined # this breaks build
-	CFLAGS += -fsanitize-trap
-	ifeq ($(CC),clang)
-		CFLAGS += -Wunused-variable
-	endif
-endif
-
 ### X11 UI
 ifeq ($(WITH_X11), yes)
 	X11CFLAGS = $(shell pkg-config --cflags x11 xext) -D_GNU_SOURCE=1
 	X11LIBS = $(shell pkg-config --libs x11 xext)
 
-	CFLAGS += $(X11CFLAGS) -DHAS_X11=1
+	override CFLAGS += $(X11CFLAGS) -DHAS_X11=1
 	LIBS += $(X11LIBS)
 	DOTOS += src/ui_x11.o
 endif
@@ -109,7 +104,7 @@ ifeq ($(WITH_SDL), yes)
 	SDLCFLAGS = $(shell pkg-config --cflags SDL_gfx sdl12_compat)
 	SDLLIBS = $(shell pkg-config --libs SDL_gfx sdl12_compat)
 
-	CFLAGS += $(SDLCFLAGS) -DHAS_SDL=1
+	override CFLAGS += $(SDLCFLAGS) -DHAS_SDL=1
 	LIBS += $(SDLLIBS)
 	DOTOS += src/ui_sdl.o
 endif