lrelease should be a required build dependency

[felixonmars]

When lrelease is not present, the cmake step generates a warning instead of an error, and proceeding with make will result in the following error:

[ 25%] Generating danmaQ.1
/bin/sh: LRELEASE-NOTFOUND: command not found
make[2]: *** [src/translations/CMakeFiles/translations.dir/build.make:63: /build/danmaq/src/danmaQ-0.2.3/src/translations/zh_CN.qm] Error 127
make[2]: *** Waiting for unfinished jobs....
/bin/sh: LRELEASE-NOTFOUND: command not found
make[2]: *** [src/translations/CMakeFiles/translations.dir/build.make:71: /build/danmaq/src/danmaQ-0.2.3/src/translations/ja_JP.qm] Error 127
/bin/sh: LRELEASE-NOTFOUND: command not found
make[2]: *** [src/translations/CMakeFiles/translations.dir/build.make:67: /build/danmaq/src/danmaQ-0.2.3/src/translations/zh_TW.qm] Error 127
make[1]: *** [CMakeFiles/Makefile2:216: src/translations/CMakeFiles/translations.dir/all] Error 2

This should be caught during the cmake process.