grub-install: error: /usr/lib/grub/i386-pc/ doesn't exist.

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Error, as found on a BIOS/GPT system:

# grub-install /dev/sda
grub-install: error: /usr/lib/grub/i386-pc/ doesn't exist. Please specify --target or --directory.

Further confused by:

# ls -d /usr/lib/grub/x86_64-emu/
# grub-install /dev/sda  --directory=/usr/lib/grub/x86_64-emu --target=i386-pc
grub-install: error: Unknown platform `x86_64-emu'.

On a BIOS Gentoo system, it is not necessary to set the GRUB_PLATFORMS in /etc/portage/make.conf.
Remove GRUB_PLATFORMS, and emerge grub again:

# emerge --ask --newuse --deep sys-boot/grub:2
[ebuild   R    ] sys-boot/grub-2.02_beta3-r1  GRUB_PLATFORMS="efi-64* pc* -emu*"

# grub-install     /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.