Fix check for qmake.
qmake isn't always named 'qmake'. kdesrc-build searches for qmake before
it runs to avoid having to take a long time to fail to build dozens of
modules, but should search for the same names looked for by the FindQt4
In addition to correcting the check, I've also fixed the usage of
'qmake' in the QMakeBuildSystem. Thanks for the prompt bug report! (And
please re-open if this fails for you, it's a bit more difficult for me
to thoroughly test)