Move grabbers configuration from defs.h to configure-grabbers.prf

On clean rebuild in QtCreator I have some annoying messages like this:

...
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: libgrab.a(QtGrabberEachWidget.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: libgrab.a(QtGrabber.o) has no symbols
...

because these *.cpp files contain preprocessor guards and the preprocessed *.cpp doesn't contain any code.
So I decided to move grabbers configuration from common/defs.h header.
It is possible, because the set of available grabbers depends only on current platform.
I think that project files should manage which sources will take part in the build process, but not the preprocessor.