gsettings.m4: @GSETTINGS_RULES@: Support srcdir != builddir with nonrecursive make

In hotssh I use nonrecursive make.  gnome-continuous uses srcdir !=
builddir by default.  @GSETTINGS_RULES@ will then attempt to touch a
nonexistent path.

This patch fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=712171