Michael Catanzaro authored Jul 22, 2019 and Guido Gunther committed Aug 12, 2019

When using pkg-config variables to get installation directory locations,
we must be careful to redefine any pkgconfig variables necessary to
ensure the installation does not occur outside the build prefix. We
could do, e.g.:

module_dir = gladeui_dep.get_pkgconfig_variable(
    'moduledir',
    define_variable: ['libdir', libdir)])

Except in the particular case of glade, this is not possible because the
pkg-config file uses absolute paths (i.e. the moduledir variable is not
relative to libdir, and not even relative to prefix!) so there's no way
to override it. This means we can't use the pkg-config directories at
all.

If bored, fixing glade might be fun to do. In the meantime, we'll need
to install these manually. This fixes the files being installed under
/usr instead of /app when building a Fedora flatpak.

See https://gitlab.gnome.org/GNOME/glade/issues/379.