SCons: Generate header with info on which modules are enabled

We already had `MODULE_*_ENABLED` defines but only in the modules
environment, and a few custom `*_ENABLED` defines in the main env
when we needed the information in core.

Now this is defined in a single header which can be included in the
files that need this information.

modules/modules_builders.py

@@ -0,0 +1,16 @@
+"""Functions used to generate source files during build time
+
+All such functions are invoked in a subprocess on Windows to prevent build flakiness.
+"""
+
+from platform_methods import subprocess_main
+
+
+def generate_modules_enabled(target, source, env):
+    with open(target[0].path, 'w') as f:
+        for module in env.module_list:
+            f.write('#define %s\n' % ("MODULE_" + module.upper() + "_ENABLED"))
+
+
+if __name__ == '__main__':
+    subprocess_main(globals())