cpp,qt: Reduce risk of typos in preprocessor conditions
* configure.ac: Add -Wundef to CPPFLAGS in maintainer mode. Explicitly define some macros as 0 instead of leaving them undefined. * qt/capslock.cpp, qt/capslock_p.h, qt/capslock_unix.cpp, qt/main.cpp, qt5/capslock.cpp, qt5/capslock_p.h, qt5/capslock_unix.cpp: Use #if instead of #ifdef to check whether a feature should be enabled.
Using -Wundef in combination with #if (instead of #ifdef) should help
reduce the risk of typos in macro identifiers (see previous commit).