Better detection of clang in ./configure. Issue #7678.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Make sure ./configure tests valid C99 programs. Issue #7678.

Clang gives a big fat warning that there's no return value for the
statement, since the prototype defaults to 'int'.

Signed-off-by: Austin Seipp <aseipp@pobox.com>