[Qt] Skip the build with a warning if certain tools cannot be found in the PATH
https://bugs.webkit.org/show_bug.cgi?id=110215

Reviewed by Tor Arne Vestbø.

Check that gperf, python, ruby, perl, bison and flex are in the PATH before
continuing the build. Otherwise skip with an error message explaining which
programs are missing from the build.

* qmake/mkspecs/features/configure.prf:
* qmake/mkspecs/features/functions.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143326 268f45cc-cd09-0410-ab3c-d52691b4dbfc