Move G_DIR_SEPARATOR* and G_SEARCHPATH_SEPARATOR* into glibconfig.h

As platform-dependent macros, they belong in glibconfig.h.
This also makes it one less place where g-ir-scanner picks definitions
from the wrong ifdef branch; see
https://bugzilla.gnome.org/show_bug.cgi?id=696935

Meson configuration support is also added in this commit.

https://bugzilla.gnome.org/show_bug.cgi?id=757284