libcharset: Don't try to include configmake.h

It's an internal gnulib thing which will never be available while
building glib. Always expect LIBDIR and fallback to using that for
compatibility with the existing MSVC projects:
https://mail.gnome.org/archives/gtk-devel-list/2017-December/msg00000.html

https://bugzilla.gnome.org/show_bug.cgi?id=346816