Stop including g++-unix.conf in the freebsd-clang mkspec.

Most of the settings there end up overwritten by the clang.conf include
that comes afterwards, except for a few things such as QMAKE_LINK_C,
which remains set to "gcc" and breaks things when one uses
CONFIG=use_c_linker.

QMAKE_LFLAGS_NOUNDEF was coming from g++-unix.conf, though, so we now
manually set it in freebsd-clang's qmake.conf.

Change-Id: Ibd16f59d43eb19e72adf4919da9ce3007100b60f
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>