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.