Add check for "essential" programs before building.

If you (for whatever reason) don't have things like cmake installed before
trying to run a full build you'll just spend a lot of time to eventually figure
out that kdesrc-build wasn't able to build anything.

Now we can (optionally!) skip the build process entirely if "essential"
programs do not seem to be available before running the build. Each build
system type is able to determine which programs are considered essential.

Note that if 'qmake' is the only holdup then we don't complain if we seem to be
building Qt or if we're in pretend mode (since it's certainly possible to fool
build system detection when in pretend mode). If qmake is really missing the
user /should/ just find out when they run without --pretend (since then Qt
should show up amongst the build-modules' build systems).

BUG:263936
FIXED-IN:1.15