check_gnu_make

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.

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:

     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.

AC_DEFUN(
        [CHECK_GNU_MAKE], [ AC_CACHE_CHECK( for GNU make,_cv_gnu_make_command,
                _cv_gnu_make_command='' ;
dnl Search all the common names for GNU make
                for a in "$MAKE" make gmake gnumake ; do
                        if  ( $a --version 2> /dev/null | grep  -q GNU  ) ;  then
                                _cv_gnu_make_command=$a ;
                                break;
                        fi
                done ;
        ) ;
dnl If there was a GNU version, then set @ifGNUmake@ to the empty string, '#' otherwise
        if test  "x$_cv_gnu_make_command" != "x"  ; then
                ifGNUmake='' ;
        else
                ifGNUmake='#' ;
        fi
        AC_SUBST(ifGNUmake)
] )