configure.ac: check for libexpat when no pkg-config is available
configure.ac: check for libexpat when no pkg-config is available

Previously, when no pkg-config was available for
libexpat we would just add the needed linking
flags without any extra check.

Now, we check that the library and the headers are
also installed in the building environment.

Reviewed-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler