always use our own internal assertion message symbol

Re-using glibc's __abort_msg symbol causes linking problems, since the symbol
is declared private. Always use our own__glib_abort_msg symbol to store
assertion messages, to avoid compatibility and linking problems.

Also fix the test case to work with out of tree builds (such as "make
distcheck"), and re-enable it.

https://bugzilla.gnome.org/show_bug.cgi?id=594872