Renamed CMake option OGS_DONT_USE_OPTIONAL_SUBMODULES to OGS_USE_OPTIONAL_SUBMODULES.

Searching for optional submodules has to be actively enabled by this
option. This prevents unexpected CMake errors when developers checked
out the submodules without their knowledge (as it happens e.g. with
the Windows GitHub app).

ThirdParty/CMakeLists.txt

@@ -11,16 +11,16 @@ ADD_SUBDIRECTORY(zlib)
 ADD_SUBDIRECTORY(gtest)
 
 # This is a workaround for Travis builds.
-OPTION(OGS_DONT_USE_OPTIONAL_SUBMODULES "Option for disabling optional submodules" OFF)
+OPTION(OGS_USE_OPTIONAL_SUBMODULES "Option for enabling optional submodules" OFF)
 
 # VtkFbxConverter
 IF(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/VtkFbxConverter/CMakeLists.txt
-	AND NOT OGS_DONT_USE_OPTIONAL_SUBMODULES)
+	AND OGS_USE_OPTIONAL_SUBMODULES)
 	ADD_SUBDIRECTORY(VtkFbxConverter)
 ENDIF()
 
 # VtkOsgConverter
 IF(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/VtkOsgConverter/CMakeLists.txt
-	AND NOT OGS_DONT_USE_OPTIONAL_SUBMODULES)
+	AND OGS_USE_OPTIONAL_SUBMODULES)
 	ADD_SUBDIRECTORY(VtkOsgConverter)
 ENDIF()
\ No newline at end of file