Use "marblewidget" target again instead of var ${MARBLEWIDGET}

Summary:
It is just the targetname, use target property OUTPUT_NAME to control
the  actual library name

Reviewers: rahn, sanjibanb, shentey, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2366

# Conflicts:
#	tools/mbtile-import/CMakeLists.txt