giscanner: fix --no-libtool on macOS

Currently g-ir-scanner fails on macOS when invoked in --no-libtool mode
if the library it links to has not been installed into its destination
(which is likely to be the case). In libtool mode this issue does not
occur as DYLD_LIBRARY_PATH will get set by libtool in an appropriate
manner. This patch ensures DYLD_LIBRARY_PATH will get set.

This patch is also essential for meson and cmake to generate
gobject-introspection bindings.

More information can be found e.g. at
https://mail.gnome.org/archives/gtk-osx-users-list/2018-February/msg00000.html,
and the github repo that was produced subsequently at
https://github.com/wagavulin/gir-mac-sample.
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