A direct clone now gives you dev which is future Qt 6, none of the
code using Qt expects Qt6 yet
Using 5.15 now (instead 5.12 before) means we have to disable
sanitize=vptr in parts of qt in kimageformats since those
parts now compile with no-rtti
While at it make the compilation script a bit better:
* Don't need to disable compressing rcc files, only needed to pass
the CFLAGS to the QMAKE_CFLAGS
* Also fix the "make qmake faster" sed command
tsdgeos