[ConfigParser] Don't warn about deprecated configparser.hh when compiling

configparser.cc.

Rationale: don't spam the user with warnings he can do nothing about.  He will
still get warnings when he uses the ConfigParser in is his own programs, and
we (as developers) will have to remove configparser.cc the same instant we
remove configparser.hh anyway.

Merged from trunk@6356.

[[Imported from SVN: r6358]]