link libmesa_util.a users with --gc-sections
avoids unwanted external references in shared libraries:
_ZNKSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE4findEcm
_ZNKSt3__121__basic_string_commonILb1EE20__throw_length_errorEv
_ZdlPv
_Znwm
__gxx_personality_v0
follows what the meson build does
problem reported by sthen@ when building sd2-ttf on i386
While we no longer install .la files for Mesa naddy@ pointed out that
using libtool to build something like libGLU could result in .la files
which reference old Mesa .la files.
Remove Mesa .la files found in DESTDIR on install as suggested by
millert@
ok millert@ matthieu@ deraadt@