##### http://autoconf-archive.cryp.to/_ac_c_ifdef.html
#
# SYNOPSIS
#
#   _AC_C_IFDEF(MACRO-NAME, ACTION-IF-DEF, ACTION-IF-NOT-DEF)
#
# DESCRIPTION
#
#   Check for the definition of macro MACRO-NAME using the current
#   language's compiler.
#
# LAST MODIFICATION
#
#   2004-09-07
#
# COPYLEFT
#
#   Copyright (c) 2004 Ludovic Courtès <ludo@chbouib.org>
#
#   Copying and distribution of this file, with or without
#   modification, are permitted in any medium without royalty provided
#   the copyright notice and this notice are preserved.

AC_DEFUN([_AC_C_IFDEF],
  [AC_COMPILE_IFELSE([#ifndef $1
                      # error "Macro $1 is undefined!"
		      /* For some compilers (eg. SGI's CC), #error is not
		         enough...  */
		      please, do fail
		      #endif],
		     [$2], [$3])])