Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/posix_thread.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_POSIX_THREAD)
+
 #include <assert.h>
 #include <string.h>
 
@@ -103,3 +105,5 @@ void _glfwPlatformUnlockMutex(_GLFWmutex* mutex)
     pthread_mutex_unlock(&mutex->posix.handle);
 }
 
+#endif // GLFW_BUILD_POSIX_THREAD
+