package/gobject-introspection: fix host-linking

When building, gobject-introspection uses tools/g-ir-scanner to build
several .gir and .typelib files. To that goal, it internally builds and
runs a few small executables linked to libglib2. However, it does not
pass them any LDFLAGS that we could pass the buildsystem. So, it either
ends up trying to link with the system's libglib2, which may not be
instaleld (build breaks), or is installed to an other version (build may
break); in either cases, this is not good...

g-ir-scanner can use the argument --lib-dirs-envvar to pass a list of
library directories to search for. However, during the build process,
this is not possible due to the build process calling g-ir-scanner
directly without letting the user (us) pass any option.

When discussing with upstream, they explained that the only solution in
that case was to set and export LD_LIBRARY_PATH to point to the location
where the correct libglib2 was installed.

Ergo, that's what we do.

This fix has the added benefit of allowing the host gobject-introspection
to build the host .gir, .rnc, and .typelib files, which some packages
may require.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>