Generate config.h

This allows us to remove a lot of #ifdef's. Either we want to use
a config.h or we don't. Since we'll want it for e.g. optional gsound
support later on let's have it.

src/config.h.in

@@ -0,0 +1,4 @@
+/*
+ * Autogenerated by the Meson build system.
+ * Do not edit, your changes will be lost.
+ */