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_module.c

@@ -28,6 +28,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_POSIX_MODULE)
+
 #include <dlfcn.h>
 
 //////////////////////////////////////////////////////////////////////////
@@ -49,3 +51,5 @@ GLFWproc _glfwPlatformGetModuleSymbol(void* module, const char* name)
     return dlsym(module, name);
 }
 
+#endif // GLFW_BUILD_POSIX_MODULE
+