Fix ebuilds

Apart from dropping the mesa use flag, this should fix the following issues:

# emerge -av1 retroarch

These are the packages that would be merged, in order:

Calculating dependencies... done!

!!! All ebuilds that could satisfy "retroarch" have been masked.
!!! One of the following masked packages is required to complete your request:
- games-emulation/retroarch-9999-r3::menelkir (masked by: package.mask, missing keyword, invalid: DEPEND: USE flag 'mesa' referenced in conditional 'mesa?' is not in IUSE, invalid: RDEPEND: USE flag 'mesa' referenced in conditional 'mesa?' is not in IUSE)
- games-emulation/retroarch-1.9.13-r2::menelkir (masked by: package.mask, invalid: DEPEND: USE flag 'mesa' referenced in conditional 'mesa?' is not in IUSE, invalid: RDEPEND: USE flag 'mesa' referenced in conditional 'mesa?' is not in IUSE)
- games-emulation/retroarch-1.9.7-r2::menelkir (masked by: invalid: DEPEND: Missing ')' at end of string, invalid: RDEPEND: Missing ')' at end of string)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.