FreeBSD/Linux Kernel Cross Reference
sys/contrib/libsodium/m4/ax_check_gnu_make.m4

     # ===========================================================================
     #    https://www.gnu.org/software/autoconf-archive/ax_check_gnu_make.html
     # ===========================================================================
     #
     # SYNOPSIS
     #
     #   AX_CHECK_GNU_MAKE()
     #
     # DESCRIPTION
    #
    #   This macro searches for a GNU version of make. If a match is found:
    #
    #     * 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 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`.
    #
    #   Here is an example of its use:
    #
    #   Makefile.in might contain:
    #
    #     # A failsafe way of putting a dependency rule into a makefile
    #     $(DEPEND):
    #             $(CC) -MM $(srcdir)/*.c > $(DEPEND)
    #
    #     @ifGNUmake@ ifeq ($(DEPEND),$(wildcard $(DEPEND)))
    #     @ifGNUmake@ include $(DEPEND)
    #     @ifGNUmake@ endif
    #
    #   Then configure.in would normally contain:
    #
    #     AX_CHECK_GNU_MAKE()
    #     AC_OUTPUT(Makefile)
    #
    #   Then perhaps to cause gnu make to override any other make, we could do
    #   something like this (note that GNU make always looks for GNUmakefile
    #   first):
    #
    #     if  ! test x$_cv_gnu_make_command = x ; then
    #             mv Makefile GNUmakefile
    #             echo .DEFAULT: > Makefile ;
    #             echo \  $_cv_gnu_make_command \$@ >> Makefile;
    #     fi
    #
    #   Then, if any (well almost any) other make is called, and GNU make also
    #   exists, then the other make wraps the GNU make.
    #
    # LICENSE
    #
    #   Copyright (c) 2008 John Darrington <j.darrington@elvis.murdoch.edu.au>
    #   Copyright (c) 2015 Enrico M. Crisostomo <enrico.m.crisostomo@gmail.com>
    #
    #   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. This file is offered as-is, without any
    #   warranty.
    
    #serial 9
    
    AC_DEFUN([AX_CHECK_GNU_MAKE],dnl
      [AC_PROG_AWK
      AC_CACHE_CHECK([for GNU make],[_cv_gnu_make_command],[dnl
        _cv_gnu_make_command="" ;
    dnl Search all the common names for GNU make
        for a in "$MAKE" make gmake gnumake ; do
          if test -z "$a" ; then continue ; fi ;
          if "$a" --version 2> /dev/null | grep GNU 2>&1 > /dev/null ; then
            _cv_gnu_make_command=$a ;
            AX_CHECK_GNU_MAKE_HEADLINE=$("$a" --version 2> /dev/null | grep "GNU Make")
            ax_check_gnu_make_version=$(echo ${AX_CHECK_GNU_MAKE_HEADLINE} | ${AWK} -F " " '{ print $(NF); }')
            break ;
          fi
        done ;])
    dnl If there was a GNU version, then set @ifGNUmake@ to the empty string, '#' otherwise
      AS_VAR_IF([_cv_gnu_make_command], [""], [AS_VAR_SET([ifGNUmake], ["#"])],   [AS_VAR_SET([ifGNUmake], [""])])
      AS_VAR_IF([_cv_gnu_make_command], [""], [AS_UNSET(ax_cv_gnu_make_command)], [AS_VAR_SET([ax_cv_gnu_make_command], [${_cv_gnu_make_command}])])
      AC_SUBST([ifGNUmake])
    ])

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