Silence aclocal warnings

aclocal -I includes/
configure.ac:37: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:193: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2590: _AC_COMPILE_IFELSE is expanded from...
../../lib/autoconf/general.m4:2606: AC_COMPILE_IFELSE is expanded from...
../../lib/m4sugar/m4sh.m4:639: AS_IF is expanded from...
../../lib/autoconf/general.m4:2031: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2052: AC_CACHE_CHECK is expanded from...
includes/egg_acinclude.m4:41: EGG_CHECK_CCWALL is expanded from...
configure.ac:37: the top level
Detect and use -fcolor-diagnostics if available (clang)