Starting with autoconf 2.70, obsolescence warnings are output by
default. To keep the script as compatible as possible, I build
autoconf 2.60 and ran the autoupdate that came with it on
configure.ac and all m4 files.
Autoconf detects the Clang compiler as GNU GCC (clang sets defines like
__GNUC__ etc) but Clang is *not* completely compatible. If we detect
Clang we set ac_vc_c_compiler_gnu to 'no'.