How to test if preprocessor defines foobar in macro?

[Dr. David Kirkby]

I'm trying to modify this macro

http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_count_cpus.m4

which attempt to get the number of CPUs in a system. I'd like to extend 
it to cover Solaris, AIX and HP-UX.

It seems sensible to me to only do a test on a platform it will work on, 
so I think thinking of something like

if `uname` = "HP-UX"
  write test for HP-UX
fi


or I could check if __hpux__ is defined or not.

Can anyone show me how to modify the above autoconf macro to do the test 
using either uname, a macro that the pre-processor defines, or other 
that allows me to execute a bit of code on only one platform. method.


There's another macro I've seen used for determine cache sizes of 
processors.

http://www.nongnu.org/autoconf-archive/ax_cache_size.html

which depends on the CPU being an x86 one, which it will often not be. I 
don't fancy the task of trying to work out cache sizes for every 
processor used on machines running Solaris, AIX or HP-UX, so will try to 
modify that so it returns a sensible guess (say 1 MB) on processors it 
does not know about. The test is causing a script to crash on HP-UX.

Dave