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_time.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_POSIX_TIMER)
+
 #include <unistd.h>
 #include <sys/time.h>
 
@@ -61,3 +63,5 @@ uint64_t _glfwPlatformGetTimerFrequency(void)
     return _glfw.timer.posix.frequency;
 }
 
+#endif // GLFW_BUILD_POSIX_TIMER
+