Fix localization files' installation on Linux/MacOS
Use $PREFIX/share/goldendict for locale files per Linux conventions

There is no need to use $PREFIX/share/*apps*/goldendict.
Linux: Better detection of installation prefix

1. qmake PREFIX=/usr

or

2. PREFIX=/usr qmake

or

3. export PREFIX=/usr; qmake

All should work.

Previously, the most typical #1 was not working.