enable --enable-gcc-warnings by default in a from-clone build

[Jim Meyering]

Eric did this in libvirt, and I liked it, so also in coreutils.
Finally, I'm updating all other C projects I tend to do the same thing:

>From d3a81337ff149294b7cf63c45a61ffa24fa542c5 Mon Sep 17 00:00:00 2001
From: Eric Blake <address@hidden>
Date: Tue, 23 Oct 2012 11:07:29 +0200
Subject: [PATCH] build: default to --enable-gcc-warnings in a git tree

Anyone building from cloned sources can be assumed to have a new
enough environment, such that enabling gcc warnings by default will
be useful.  Tarballs still default to no warnings, and the default
can still be overridden with --disable-gcc-warnings.
* configure.ac (gl_gcc_warnings): Set default based on environment.
---
 configure.ac | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 832fc20..417f325 100644
--- a/configure.ac
+++ b/configure.ac
@@ -187,7 +187,11 @@ AC_ARG_ENABLE([gcc-warnings],
      *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
    esac
    gl_gcc_warnings=$enableval],
-  [gl_gcc_warnings=no]
+  [if test -d "$srcdir"/.git; then
+     gl_gcc_warnings=yes
+   else
+     gl_gcc_warnings=no
+   fi]
 )

 if test "$gl_gcc_warnings" = yes; then
--
1.8.0