cmake (gmsh) : add an option to disable ctest (so that I can use ctest

in dg and still include gmsh CMakeList.txt)

CMakeLists.txt

@@ -1760,6 +1760,8 @@ endif(ENABLE_WRAP_JAVA)
 
 include(CPack)
 
+if(NOT DISABLE_GMSH_TESTS)
+  #disabling tests is usefull when including this CMakeLists in an external project
   include(CTest)
   file(GLOB_RECURSE TESTFILES 
        tutorial/*.geo demos/*.geo benchmarks/?d/*.geo benchmarks/extrude/*.geo)
@@ -1778,6 +1780,7 @@ endforeach()
   #     # sys.path.insert(0, "path/to/your/package")
   #   endforeach()
   # endif(HAVE_PYTHON)
+endif(NOT DISABLE_GMSH_TESTS)
 
 message(STATUS "")
 message(STATUS "Gmsh ${GMSH_VERSION} has been configured for ${GMSH_OS}")