[Libreoffice] making it easy to valgrind the smoketest etc.

[Caolán McNamara]

So to make it easy to valgrind the smoketest and soffice, if you now

export VALGRIND=memcheck
make check

the soffice instance will be run under valgrind and error out if
valgrind detects any errors during the smoketest.

I took VALGRIND=something because that's what we do in hunspell as well
during its make check which is part of the standard libreoffice build so
doing an export VALGRIND=memcheck before the build runs the hunspell
regression test under valgrind as well as the smoketest/LibreOffice
itself.

C.