Some mozconfigs don't include mozconfig.linux*, and don't get gtk-related
definitions, so move them in a separate mozconfig. To avoid having two
files, one for 32-bit builds and one for 64-bit builds, rely on the
includer to set PKG_CONFIG_LIBDIR appropriately.
At the same time, move all the --enable-default-toolkit=cairo-gtk2 in that
new file in the case the gtk3 package wasn't pulled from tooltool.
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