Better configure checks to see if GCC can be built

Until now, to test if GCC can use `sys/cdefs.h', we were building
a small test program using it. But after testing on an Ubuntu 14.04,
we noticed that the GCC test during the configure script passes, but
GCC still can't be built. After some investigation we noticed its
available in other directories, but during the build of GCC, those
directories aren't used, and it only assumes it to be under
`/usr/include'. So with this commit, we are only checking this
particular location for this header, not a test run of GCC.

After fixing this, we noticed that GCC's build crashed again because
it couldn't link with `libc.a' (or `libc.so'). So we also added a
for this library and added a new warning to inform the user what they
might be able to do.

Finally, we noticed that in one of the last steps of building GCC,
we weren't using `&&', but `;', so the GCC name file would be
built, even when the GCC build failed.

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