Build specs on OS X should not be hard-coded to macx-g++

[Tzu-ping Chung]

Newer versions of Qt provides spec unsupported-macx-libc++, which uses LLVM's libc++ instead of libstdc++. Current configure script always set the build spec to macx-g++, which is not compatible with this new spec. There should be a way to change the spec parameter—at least honour global environment variables, e.g.

if [ -z "$QMAKE_PARAMS" ]; then
    QMAKE_PARAMS="-spec macx-g++"
fi