Don't build ovito executable for Python-only packages

src/CMakeLists.txt

@@ -36,7 +36,7 @@ IF(OVITO_CMAKE_FILE_INJECTION_PLUGINS)
 ENDIF()
 
 # Build the main application executable 'ovito'.
-IF(OVITO_BUILD_GUI)
+IF(OVITO_BUILD_GUI AND NOT OVITO_BUILD_PYTHON_PACKAGE)
 	ADD_SUBDIRECTORY(ovito_exe)
 ENDIF()