On some systems, there is a /usr/bin/lrelease wrapper even if the real
lrelease program is not installed. So let's try to execute lrelease
after it has been found by find_program to see if it is really working.
Define generate_translations_header as an external project to be able
to use the compilation toolchain for the host instead of the toolchain
for the target.
If a translation file exists in a "translations" directory located in
the same directory as the binary, it is used in priority (this can be
useful when working on translations as you don't have to recompile the
whole program all the time), and if no such file is found the embedded
translation file is used (if it exists).
