configure.ac: Do not use Pthreads with MinGW.
configure.ac: Do not use Pthreads with MinGW.

Match the behavior of the SCons MinGW build.

This patch also fixes these build errors.

  CC       glapi_entrypoint.lo
glapi_entrypoint.c: In function 'init_glapi_relocs_once':
glapi_entrypoint.c:341:4: error: unknown type name 'pthread_once_t'
    static pthread_once_t once_control = PTHREAD_ONCE_INIT;
    ^
glapi_entrypoint.c:341:41: error: 'PTHREAD_ONCE_INIT' undeclared (first use in this function)
    static pthread_once_t once_control = PTHREAD_ONCE_INIT;
                                         ^
glapi_entrypoint.c:341:41: note: each undeclared identifier is reported only once for each function it appears in
glapi_entrypoint.c:342:4: error: implicit declaration of function 'pthread_once' [-Werror=implicit-function-declaration]
    pthread_once( & once_control, init_glapi_relocs );
    ^

Signed-off-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer