configure.ac: Check and use -Wno-unused-local-typedefs GCC option

With GCC 4.8.2 the COMPILE_TIME_ASSERT macro produces a spurious
warning about an unused local typedef:

    In file included from pixman.c:29:0:
    pixman.c: In function 'optimize_operator':
    pixman-private.h:1019:22: warning: typedef 'compile_time_assertion' locally defined but not used [-Wunused-local-typedefs]

The flag -Wno-unused-local-typedefs suppresses that warning.