Import upstream patch to fix the detection of clang's version number.

Our clang reports itself as "FreeBSD clang version x.y.z" instead of just
"clang version x.y.z", which was preventing the sed pattern used in the
configure script from matching and thus QT_CLANG_MAJOR_VERSION and
QT_CLANG_MINOR_VERSION were never defined. One of the consequences is that
tests for those values in mkspecs later always failed, which led to some
features such as C++14 support being disabled.

PR: 210327
MFH: 2016Q3