SlackBuildsOrg/system/refind

rEFInd (An EFI boot manager utility)

A graphical boot manager for EFI- and UEFI-based computers, such as all
Intel-based Macs and recent (most 2011 and later) PCs. rEFInd presents
a boot menu showing all the EFI boot loaders on the EFI-accessible
partitions, and optionally BIOS-bootable partitions on Macs.
EFI-compatible OSes, including Linux, provide boot loaders that rEFInd
can detect and launch. rEFInd can launch Linux EFI boot loaders such
as ELILO, GRUB Legacy, GRUB 2, and 3.3.0 and later kernels with EFI
stub support.

This script does not build the boot manager from source, in stead it uses
the pre-build binary then convert to Slackware's.