Do not remove -Wp, before -D and -U when recording used compiler flags

This breaks the pattern used when undefining and then defining a
variable in the same flag:

$ echo | g++ -Wp,-UVAR,-DVAR=1 -c -x c++ -

Here -Wp,-UVAR,-DVAR=1 will first undefine VAR and then define it to 1

If the -Wp, is dropped, this fails:

$ echo | g++ -UVAR,-DVAR=1 -c -x c++ -
<command-line>: warning: extra tokens at end of #undef directive

Now it will only undefine VAR, but not define it. I.e.

instead of

 #undef VAR
 #define VAR 1

you get

 #undef VAR,-DVAR=1

which explains the rather cryptic warning about extra token after #undef.

cmake/modules/RootConfiguration.cmake

@@ -729,7 +729,7 @@ if (cxxmodules)
 endif()
 
 string(REGEX REPLACE "(^|[ ]*)-W[^ ]*" "" __fflags "${CMAKE_Fortran_FLAGS}")
-string(REGEX MATCHALL "-(D|U)[^ ]*" __defs "${CMAKE_CXX_FLAGS}")
+string(REGEX MATCHALL "(-Wp,)?-(D|U)[^ ]*" __defs "${CMAKE_CXX_FLAGS}")
 set(ROOT_COMPILER_FLAG_HINTS "#
 set(ROOT_DEFINITIONS \"${__defs}\")
 set(ROOT_CXX_FLAGS \"${__cxxflags}\")