Fix install_desktop_files non-portability and add out-of-tree support.

Prepend $(srcdir)/ to the sources in install commands, and get rid of
the -T flag, as it's GNU coreutils install-specific; some versions of
install don't support -T at all, and FreeBSD's install has a -T flag
that takes an argument and has a completely different meaning.

Fixes bug 10292; fix came from

    http://marc.info/?l=openbsd-ports&m=140571104528980&w=2

Change-Id: I8e80d475a728a17848736be043f97d2b90a0be82
Reviewed-on: https://code.wireshark.org/review/3107
Reviewed-by: Guy Harris <guy@alum.mit.edu>