C++ PATCH to allow &main w/o -pedantic

[Jason Merrill]

Rebuilding Fedora with the current branch turned up a lot of configury 
failures due to C++ package autoconf scripts doing things like

int main() { main(); return 0; }

in AC_CHECK_LIB tests because they can't call any other functions 
without a prototype.  This is bogus, but it doesn't seem worth breaking 
all these packages just to make a point.  Previous releases (including 
4.3) only complained with -pedantic; this patch restores that behavior.

Tested x86_64-pc-linux-gnu, applied to trunk.