Make qmake support dynamic spec selection

If you don't pass a -spec argument to qmake, the default target spec is used. In order to support host builds outside of qtbase when cross-compiling, there should be a way to select the host spec instead. For example, qmake could recognize a magic comment in the .pro file itself:

#pragma spec:default-host
TARGET=lrelease
...

Such a generic solution would also allow us to get rid of the hard-coded spec selection in the configure script (which tends to grow stale).