Providing pkg-config variables for rcc, lupdate, lrelease binaries, like with moc and uic

Pkg-config is very convenient when compiling a project without qmake.

In many platforms, qt build binaries (moc, uic, rcc...) are prefixed or suffixed, for example debian suffixes them with -qt4 if qt3 is already installed and macports suffix them with -mac distinguish native and x based port. So the prefix available on the pkg-config files often is not enough to locate the binary.

pkg-config provides two convenient variables moc_location and uic_location (at least in debian packaging). It would be nice having similar variables for other binaries such as rcc, lupdate and lrelease.

This will enable build tools appart from qmake to use pkg-config to locate all the binaries. I am maintaining the SCons tool for Qt4 and now it fails to identify such binaries in some situations like the above. Being able to rely on such variables will solve this issue and it will simplify the tool itself.