[R-SIG-Mac] configure.in on Mac

[cstrato]

Dear all

I would like to add to my configure.in file the following commands,
which should only be checked on Mac OS X:

   if test -z "${DYLD_LIBRARY_PATH}"; then
      echo ""
      echo "${R_PACKAGE_NAME} configuration error:"
      echo ""
      exit 1
   fi

However, these commands should only be executed on a Mac.
What do I need to do to be able to test if the configure.in file is 
executed
on a Mac?
I tried to use ${R_ARCH} but this returns an empty string.

Thank you in advance
Best regards
Christian
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