Don't try to build examples if RPATH isn't available

QMAKE_LFLAGS_RPATH is stripped from the mkspecs in official
Arch Linux Qt packages.

Don't force building examples in this case to allow qtwebengine to
build using the system Qt with a straight qmake;make.

Change-Id: Id406d55d34c0d313d69ba9f689f8f24f33038824
Reviewed-by: Andras Becsi <andras.becsi@digia.com>