configure adds warning flags not supported by all versions of gcc when $CC=gcc
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=849
Reported by @marcus
If configure detects that the C compiler used is gcc, it will add a number of warning flags. Amongst these are -Wno-long-long and -Wmultichar, which are not supported by gcc 18.104.22.168. configure should test the individual flags to see which ones are really supported.