gnu: gcc: Fix manual pages.

gnu: gcc: Fix manual pages.

Fixes bug #24069. perl is made a native input to all of the gcc-* packages
except for gcc-boot0; perl-boot0 is made a native input to gcc-final.

* gnu/packages/patches/gcc-fix-texi2pod.patch: Add patch file to fix texi2pod.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/gcc.scm (gcc-4.7, gcc-4.8, gcc-4.9, gcc-5): Use it.
(gcc-4.7)[native-inputs]: Add perl.
(gcc-4.9)[native-inputs]: Likewise.
* gnu/packages/commencement.scm (gcc-boot0)[native-inputs]: Remove perl.
(gcc-final)[native-inputs]: Add perl-boot0.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>