Stop hard coding GCC in makefiles

It is preferable to use Clang instead of GCC for its improved
diagnostics.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Add strlcat to strings.h

We define strlcat ourselves, so we should include its function prototype
in strings.h

Note that we need a way to handle sane libcs like uclibc that provide
strlcat.

Signed-off-by: Richard Yao <ryao@gentoo.org>