AX_CHECK_GNU_MAKE([run-if-true],[run-if-false])

  * The makefile variable `ifGNUmake' is set to the empty string, otherwise
    it is set to "#". This is useful for including a special features in a
    Makefile, which cannot be handled by other versions of make.
  * The makefile variable `ifnGNUmake' is set to #, otherwise
    it is set to the empty string. This is useful for including a special
    features in a Makefile, which can be handled
    by other versions of make or to specify else like clause.
  * The variable `_cv_gnu_make_command` is set to the command to invoke
    GNU make if it exists, the empty string otherwise.
  * The variable `ax_cv_gnu_make_command` is set to the command to invoke
    GNU make by copying `_cv_gnu_make_command`, otherwise it is unset.
  * If GNU Make is found, its version is extracted from the output of
    `make --version` as the last field of a record of space-separated
    columns and saved into the variable `ax_check_gnu_make_version`.
  * Additionally if GNU Make is found, run shell code run-if-true
    else run shell code run-if-false.

  # A failsafe way of putting a dependency rule into a makefile
  $(DEPEND):
          $(CC) -MM $(srcdir)/*.c > $(DEPEND)

  @ifGNUmake@ ifeq ($(DEPEND),$(wildcard $(DEPEND)))
  @ifGNUmake@ include $(DEPEND)
  @ifGNUmake@ else
  fallback code
  @ifGNUmake@ endif

  AX_CHECK_GNU_MAKE()
  AC_OUTPUT(Makefile)

  if  ! test x$_cv_gnu_make_command = x ; then
          mv Makefile GNUmakefile
          echo .DEFAULT: > Makefile ;
          echo \  $_cv_gnu_make_command \$@ >> Makefile;
  fi