configure: use AC_COMPILE_IFELSE() and AC_LANG_SOURCE() for testing flags.

configure: use AC_COMPILE_IFELSE() and AC_LANG_SOURCE() for testing flags.

That lets us completely control the program that's compiled with the
compiler flag we're testing, so we can make it a minimal program that
uses only prototype declarations and that therefore won't generate
warnings with some -W flags, e.g. -Wold-style-definition, and thus won't
falsely report those flags as unsupported.