svn 14824 broke -funsigned-char and possibly building out of tree, because

svn 14824 broke -funsigned-char and possibly building out of tree, because
$(srcdir) can't bind early since it's set to a different value in each subdir.
If it binds early, it's blank, hence an empty -I which eats the next option
as a directory to look for #include files in.

So CFLAGS has to bind late, but the check_gcc stuff should only get run once.
I added a "make V=2" mode to show when check_gcc gets run, and turned WARNINGS
into a CFLAGS line since it was always getting added anyway.