Fix: Compile on Mac OS X with the official Qt for Mac / Qt SDK for Mac.

Find translation helper binaries in QT_INSTALL_BINS. QT_INSTALL_PREFIX is
/ on OS X, since it cannot have a meaningful value, unless you are
compiling Qt to work from the build dir, in which case QT_INSTALL_PREFIX
is set to this directory.

share/qtcreator/translations/translations.pro

@@ -9,8 +9,8 @@ defineReplace(prependAll) {
     return($$result)
 }
 
-LUPDATE = $$targetPath($$[QT_INSTALL_PREFIX]/bin/lupdate) -locations relative -no-ui-lines
-LRELEASE = $$targetPath($$[QT_INSTALL_PREFIX]/bin/lrelease)
+LUPDATE = $$targetPath($$[QT_INSTALL_BINS]/bin/lupdate) -locations relative -no-ui-lines
+LRELEASE = $$targetPath($$[QT_INSTALL_BINS]/bin/lrelease)
 
 TS_FILES = $$prependAll(TRANSLATIONS, $$PWD/qtcreator_,.ts)