How can I differentiate C and C++ compiler?

["Dr. David Kirkby" <david dot kirkby at onetel dot net>]

I want a user to set environment variables CC and CXX and point them to 
C and C++ compilers. I want to check that they are in fact C and C++ 
compilers, and that CC does not point to a C++ compiler, and CXX does 
not point to a C compiler.

I've done this on HP-UX, using HP's native compiler, using:

echo "#ifdef __HP_aCC"                   >> $TESTFILE
echo "error_this_is_C++_not_C_compiler"  >> $TESTFILE
echo "#endif"                            >> $TESTFILE


${CC} -E $TESTFILE | grep error_this_is_C++_not_C_compiler >/dev/null 2>&1
if [ $? = 0 ]; then
   echo Error_C++_not_C_compiler
   rm $TESTFILE
   exit 0
fi


That works, since HP's C++ compiler defines __HP_aCC, but the C compiler 
does not. (HP's C compiler on AIX defines __HP_cc instead).


Is there anything I can do with gcc/g++ to do this? I know they both 
define __GNU__, but that does not help me.

I really only need to work for gcc/g++ >= 3.4.0, but it would handy if 
it worked for any gcc.


Dave