Replace the CFLAGS-checking code with a common macro from xine-lib.
Instead of writing custom code to check for cflags checking, import a
copy of attributes.m4 from xine-lib's repository and use the
CC_CHECK_CFLAGS macro.
The advantage lies not only in being able to reduce the custom code in
configure.ac, but also in the fact that the CC_CHECK_CFLAGS macro
caches the results, making ./configure -C quite faster on second run.
Check for the CFLAGS for any compiler and not just GCC, if the
compiler does support the flag it is better to u se it anyway,
otherwise it will be skipped.