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* configure.ac: Add -Wno-unknown-attributes if clang; otherwise
clang 3.5.0 (Fedora 21 x86-64) complains
"/usr/include/glib-2.0/glib/gmem.h: ... warning: unknown attribute
'__alloc_size__' ignored".  Use -Werror when checking for -nopie;
otherwise clang warns about -nopie instead of failing, and then
later it warns everytime the build uses -nopie.