Testing for unknown flags in different compilers

[Dale Visser]

I have been working on a changes whereby an autoconf user can invoke a macro 
like

AC_WARN_ADD([-Wextra])

to add user-specified flags to the compiler/linker invocations. Ideally we 
would like 

AC_WARN_ADD([-Wunknown-warning-flag])

to not add the offending unknown flag. My initial implementation couldn't 
detect and suppress unknown flags in the LLVM clang/clang++ compiler. Eric 
Blake pointed me to the gnulib warnings module which has a clever way utilizing 
"-Wunknown-warning-option -Werror" to handle this: 
http://lists.gnu.org/archive/html/autoconf-patches/2014-02/msg00006.html

Wanting to be thorough, I checked whether unknown flags would also get 
suppressed by the Intel C++ compiler. It turns out that AFAICT the Intel 
compiler never exits with a non-zero status for unknown flags, eliminating the 
usual way of testing in autoconf. (See also 
https://stackoverflow.com/questions/14820890/intel-compilers-silence-commandline-warnings)

I would like to take the following approach, and want to see if the community 
has any helpful advice on whether this is advisable or ways to improve on it:

IF perform test compile with given flag (e.g., -Wunknown-warning-flag) (using 
above-mentioned strategy so LLVM will detect, too) is successful(true) THEN
  Perform test compile with unknown flag, redirecting stdout and stderr to fileA
  Perform test compile without unknown flag, redirecting stdout and stderr to 
fileB
  IF "diff fileA fileB | grep --quiet -- -Wunknown-warning-flag" is successful 
(true) THEN
    Flag is unknown. Do not add to flags.
  ELSE
    Success. Add to flags. It was not explicitly mentioned in output, so must 
be known.
  FI
ELSE
  Flag is unknown. Do not add to flags.
FI

Please let me know if you see any problems or pitfalls with this approach.

Best regards,
Dale Visser