Probe for -Wno-string-plus-int.

Probe for -Wno-string-plus-int.

LLVM has its own nuts warnings about things that aren't wrong, but disabling
them with the relevant -Wno-* warning disabling command line option drives
gcc nuts because it's a command line option it doesn't recognize. (gcc 4.2.1
dies with an error. gcc 4.6 warns about it _only_ if it's warning about
something else. (PICK ONE, either you warn about this or you don't, distract
people from actual problems with noise about something clearly unrelated to
what just changed is extra-stupid.)

So just probe for it, and add the flag only if it doesn't complain about it
while we're producing an unrelated warning.