Make xerces-c, opengl, and gdal private libraries.

If these are public libraries a client build system needs to link
to them again even libcitygml was compiled into a self-contained
dynamic library.

sources/CMakeLists.txt

@@ -231,16 +231,16 @@ generate_export_header(citygml
                        EXPORT_FILE_NAME ${EXPORT_HEADER_FILE_NAME})
 
 if(XercesC_FOUND)
-  TARGET_LINK_LIBRARIES( ${target} PUBLIC XercesC::XercesC)
+  TARGET_LINK_LIBRARIES( ${target} PRIVATE XercesC::XercesC)
 else(XercesC_FOUND)
-  TARGET_LINK_LIBRARIES( ${target} PUBLIC ${XERCESC_LIBRARIES} )
+  TARGET_LINK_LIBRARIES( ${target} PRIVATE ${XERCESC_LIBRARIES} )
 endif(XercesC_FOUND)
 
 if(LIBCITYGML_USE_OPENGL)
-  TARGET_LINK_LIBRARIES( ${target} PUBLIC ${OPENGL_LIBRARIES} )
+  TARGET_LINK_LIBRARIES( ${target} PRIVATE ${OPENGL_LIBRARIES} )
 endif(LIBCITYGML_USE_OPENGL)
 if(LIBCITYGML_USE_GDAL)
-    TARGET_LINK_LIBRARIES( ${target} PUBLIC ${GDAL_LIBRARY} )
+    TARGET_LINK_LIBRARIES( ${target} PRIVATE ${GDAL_LIBRARY} )
 endif(LIBCITYGML_USE_GDAL)
 
 set_target_properties( ${target} PROPERTIES