maint: don't enable warnings by default unless GCC 4.6 or later

* configure.ac (gl_GCC_VERSION_IFELSE): New macro, from coreutils.
(gl_gcc_warnings): By default, disable warnings for GCC versions
before 4.6.  This works around a failure when building from git on
Ubuntu 10, reported privately by Nathan Stratton Treadway.

configure.ac

@@ -112,6 +112,24 @@ gl_INIT
 # paxutils modules
 tar_PAXUTILS
 
+# gl_GCC_VERSION_IFELSE([major], [minor], [run-if-found], [run-if-not-found])
+# ------------------------------------------------
+# If $CPP is gcc-MAJOR.MINOR or newer, then run RUN-IF-FOUND.
+# Otherwise, run RUN-IF-NOT-FOUND.
+AC_DEFUN([gl_GCC_VERSION_IFELSE],
+  [AC_PREPROC_IFELSE(
+    [AC_LANG_PROGRAM(
+      [[
+#if ($1) < __GNUC__ || (($1) == __GNUC__ && ($2) <= __GNUC_MINOR__)
+/* ok */
+#else
+# error "your version of gcc is older than $1.$2"
+#endif
+      ]]),
+    ], [$3], [$4])
+  ]
+)
+
 AC_ARG_ENABLE([gcc-warnings],
   [AS_HELP_STRING([--enable-gcc-warnings],
      [turn on many GCC warnings (for developers; best with GNU make)])],
@@ -120,10 +138,9 @@ AC_ARG_ENABLE([gcc-warnings],
      *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
    esac
    gl_gcc_warnings=$enableval],
-  [if test -d "$srcdir"/.git; then
-     gl_gcc_warnings=yes
-   else
-     gl_gcc_warnings=no
+  [gl_gcc_warnings=no
+   if test -d "$srcdir"/.git; then
+     gl_GCC_VERSION_IFELSE([4], [6], [gl_gcc_warnings=yes])
    fi]
 )