[CMake] add import prefix for libmapnik

so that the resulting archive is also named libmapnik

src/CMakeLists.txt

@@ -293,6 +293,7 @@ set_target_properties(mapnik PROPERTIES
     DEBUG_POSTFIX "${MAPNIK_DEBUG_POSTFIX}"
     OUTPUT_NAME "mapnik"
     PREFIX "lib"
+    IMPORT_PREFIX "lib" # for the archive on dll platforms
     VERSION ${MAPNIK_VERSION}
     # see https://github.com/mapnik/mapnik/pull/4248#issuecomment-925596509 => ABI compability only with the full version
     SOVERSION ${MAPNIK_VERSION}