cmake HAVE_PTHREAD_MUTEX_T_POINTER examples

JulNadeauCA/libagar CMakeChecks.cmake :2374

check_c_source_compiles("
#include <pthread.h>

int main(int argc, char *argv[])
{
	pthread_mutex_t mutex = NULL;
	pthread_mutex_init(&mutex, NULL);
	return (mutex != NULL);
}
" HAVE_PTHREAD_MUTEX_T_POINTER)