This package should drop -Werror enabled by the buildsystem.
From the build log we have: x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/grub-core/efiemu/runtime -DGRUB_FILE=\"efiemu/runtime/efiemu.c\" -I. -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/grub-core -I.. -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00 -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/include -I../include -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/grub-core/lib/libgcrypt-grub/include -I/var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/grub-core/lib/libgcrypt-grub/src/ -DELF32 -DAPPLE_CC -m32 -Wall -Werror -nostdlib -O2 -c -o efiemu32.o.bin /var/tmp/portage/sys-boot/grub-2.00_p5107-r1/work/grub-2.00/grub-core/efiemu/runtime/efiemu.c || exit 1; \
Seems to be limited to the "efiemu" files. These get build with TARGET_CC but not with TARGET_CFLAGS. Instead, Makefile.am hard codes the flags, ignoring the --disable-werror configure option.
Reported upstream.
Upstream seems to not care, and I can't say I really care either.