autoconf: Check asciidoc version to automatically set ASCIIDOC8

Check for asciidoc, and if it exists check asciidoc version, setting
ASCIIDOC8 when needed.  Currently it just runs asciidoc in asciidoc7
compatibility mode (see: Documentation/Makefile).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.in

@@ -23,6 +23,7 @@ VPATH = @srcdir@
 export exec_prefix mandir
 export srcdir VPATH
 
+ASCIIDOC8=@ASCIIDOC8@
 NEEDS_SSL_WITH_CRYPTO=@NEEDS_SSL_WITH_CRYPTO@
 NO_OPENSSL=@NO_OPENSSL@
 NO_CURL=@NO_CURL@

configure.ac

@@ -122,6 +122,27 @@ if test -z "$NO_TCLTK"; then
     AC_SUBST(TCLTK_PATH)
   fi
 fi
+AC_CHECK_PROGS(ASCIIDOC, [asciidoc])
+if test -n "$ASCIIDOC"; then
+	AC_MSG_CHECKING([for asciidoc version])
+	asciidoc_version=`$ASCIIDOC --version 2>&1`
+	case "${asciidoc_version}" in
+	asciidoc' '8*)
+		ASCIIDOC8=YesPlease
+		AC_MSG_RESULT([${asciidoc_version} > 7])
+		;;
+	asciidoc' '7*)
+		ASCIIDOC8=
+		AC_MSG_RESULT([${asciidoc_version}])
+		;;
+	*)
+		ASCIIDOC8=
+		AC_MSG_RESULT([${asciidoc_version} (unknown)])
+		;;
+	esac
+fi
+AC_SUBST(ASCIIDOC8)
+
 
 ## Checks for libraries.
 AC_MSG_NOTICE([CHECKS for libraries])