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.
Split acinclude.m4 in multiple macro files.
Let it be known to autoconf and aclocal to use the m4 directory.
Ignore macro files copied or linked by libtool and intltool.
