CMake: Don't use GREATER_EQUAL

It's not compatible with CMake 3.0.

dtool/src/dtoolbase/CMakeLists.txt

@@ -2,7 +2,7 @@ configure_file(pandaVersion.h.in pandaVersion.h)
 configure_file(checkPandaVersion.h.in checkPandaVersion.h)
 configure_file(checkPandaVersion.cxx.in checkPandaVersion.cxx)
 
-if(CMAKE_CXX_STANDARD GREATER_EQUAL 17)
+if(CMAKE_CXX_STANDARD GREATER 16)
   # This serves as a reminder to update checkPandaVersion.h.in when we upgrade
   # to C++17, which supports inline variables - a cleaner way of depending on
   # the Panda version symbol from a header than what we're currently doing.