perl 5.8.8: Allow perl to build when using an external toolchain.

perl 5.8.8: Allow perl to build when using an external toolchain.
This is done by allowing gcc to search for errno.h by itself instead
of manually searching for. The manual search was looking in
STAGING_INCDIR and that's not where the external toolchains headers
are. This whole test is really for handle other compilers and
operating systems, so the simple make gcc do itself should be fine
for us. Closes #1700.