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_poll.c

@@ -30,6 +30,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_POSIX_POLL)
+
 #include <signal.h>
 #include <time.h>
 #include <errno.h>
@@ -79,3 +81,5 @@ GLFWbool _glfwPollPOSIX(struct pollfd* fds, nfds_t count, double* timeout)
     }
 }
 
+#endif // GLFW_BUILD_POSIX_POLL
+