build/pkgs/info/spkg-configure.m4: check for version >= 7.0

Some distros still provide 6.x versions of GNU info, but v7.0.0
includes an important bugfix for the standalone "info" reader: it
fixes the exit code when the section you want to read does not exist.
Our singular interface now relies on this, so we require a newer
version if the system package is going to be used.

build/pkgs/info/spkg-configure.m4

@@ -1,15 +1,24 @@
 SAGE_SPKG_CONFIGURE([info], [
   AC_PATH_PROG(INFO, info)
-  AS_IF([test -z "${INFO}"], [sage_spkg_install_info=yes
-    ], [
-     dnl very old makeinfo are not texi2any, newer are symlinks to texi2any
-     AC_PATH_PROG(TEXI2ANY, texi2any)
-     AS_IF([test -z "${TEXI2ANY}"], [sage_spkg_install_info=yes
-       ], [
-        AS_IF([makeinfo -c foo 2>&1 | grep -q invalid], [
-         dnl makeinfo found, but too old, and  does not support all options that ecl likes to use
-         sage_spkg_install_info=yes])
-        rm -f stdin.info
-       ])
+  AS_IF([test -z "${INFO}"], [
+    sage_spkg_install_info=yes
+  ],[
+    AC_MSG_CHECKING([for GNU info >= 7.0])
+    info_major=$(info --version | head -n1 | cut -d' ' -f4 | cut -d. -f1)
+    AS_IF([test $info_major -lt 7], [
+      AC_MSG_RESULT([no])
+      sage_spkg_install_info=yes
+    ],[
+      AC_MSG_RESULT([yes])
+
+      AC_MSG_CHECKING([for texi2any])
+      AC_PATH_PROG(TEXI2ANY, texi2any)
+      AS_IF([test -z "${TEXI2ANY}"], [
+        AC_MSG_RESULT([no])
+	sage_spkg_install_info=yes
+      ],[
+        AC_MSG_RESULT([yes])
+      ])
     ])
+  ])
 ])