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 <vlee@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>