Configure-Time tests for presence of (required) gperf, bison and flex
are added to configure.pri and configure.json, so that we can decide
early on to skip the build, instead of starting a build that
will fail much later, yield incomprehensible error messages and
make the build of the complete Qt framework fail.

Task-number: QTBUG-52805
Change-Id: I7dab49e6e8672b72901519ad7d88de97197fe587
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>