shared_app.pri: Generalize the FreeBSD mkspecs being matched.

"freebsd-g++" restricts those common settings to a single mkspec that
matches the base system's outdated GCC 4.2 and excludes all the mkspecs for
other GCC versions and clang that are also shipped on FreeBSD.
Merge pull request #1 from rakuco/freebsd-mkspec-name

shared_app.pri: Generalize the FreeBSD mkspecs being matched.