Restrict check for obsolete macros to C++ code

Only check for the use of NULL in C++ code, not C, for example, where
it still can, and should, be used.

Also allow using "NULL" as part of a larger macro name (where it would
be followed by "_") or at the beginning/end of a string.

.github/workflows/code_checks.yml

@@ -87,8 +87,8 @@ jobs:
         run: |
             git fetch --depth=1 origin master
             if git diff origin/master \
-               ':!.github/workflows/code_checks.yml' \
-               | grep -E '^\+.*(wxOVERRIDE|wxNOEXCEPT|[^_@]NULL)'; then
+               ':**.h' ':**.cpp' \
+               | grep -E '^\+.*(wxOVERRIDE|wxNOEXCEPT|[^"_@]NULL[^"_])'; then
               echo "::error ::Please use C++11 equivalents of the deprecated macros in the new code."
               exit 1
             fi