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>

mkspecs/unsupported/freebsd-clang/qmake.conf

@@ -13,6 +13,7 @@ QMAKE_CXXFLAGS_THREAD   = $$QMAKE_CFLAGS_THREAD
 QMAKE_INCDIR            = /usr/local/include
 QMAKE_LIBDIR            = /usr/local/lib
 
+QMAKE_LFLAGS_NOUNDEF    = -Wl,--no-undefined
 QMAKE_LFLAGS_THREAD     = -pthread
 
 QMAKE_LIBS              =
@@ -28,6 +29,5 @@ QMAKE_RANLIB            =
 
 include(../../common/unix.conf)
 include(../../common/gcc-base-unix.conf)
-include(../../common/g++-unix.conf)
 include(../../common/clang.conf)
 load(qt_config)