New macro for testing whether the compiler supports a given

New macro for testing whether the compiler supports a given
command-line flag or not, and adding it to CFLAGS only if supported.
Use it for GCC flag -Wno-format-zero-length, which we were testing for
before, and for some additional flags: -Woverflow -Wstrict-overflow
-Wmissing-format-attribute -Werror=declaration-after-statement
-Wdeclaration-after-statement -Werror=variadic-macros
-Wvariadic-macros.  (The last few are because we've had some code
contributions that assumed full C99 support, and our baseline is
currently C89+, so if possible we want certain newer constructs
flagged as errors.)

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20685 dc483132-0cff-0310-8789-dd5450dbe970