build: avoid --enable-gcc-warnings on GCC <= 4.5

* configure.ac: Only enable warnings automatically when
on GCC >= 4.6 (and when building from a git checkout)
as that was the first GCC version to support fine-grained
control of warnings, allowing them to be adjusted around
certain code sections.  gnulib relies on this for certain
warnings, so avoid auto enabling this option lest we trigger
build failures on now over two year old compilers.
Reported by Zartaj Majeed with GCC 4.5.3 on cygwin.

diff --git a/configure.ac b/configure.ac
index 6504144..295802d 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -67,21 +67,6 @@ cu_have_perl=yes
 case $PERL in *"/missing "*) cu_have_perl=no;; esac
 AM_CONDITIONAL([HAVE_PERL], [test $cu_have_perl = yes])
 
-AC_ARG_ENABLE([gcc-warnings],
-  [AS_HELP_STRING([--enable-gcc-warnings],
-     [turn on many GCC warnings (for developers; best with GNU make)])],
-  [case $enableval in
-     yes|no) ;;
-     *)      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
-   fi]
-)
-
 # 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.
@@ -100,6 +85,23 @@ AC_DEFUN([gl_GCC_VERSION_IFELSE],
   ]
 )
 
+AC_ARG_ENABLE([gcc-warnings],
+  [AS_HELP_STRING([--enable-gcc-warnings],
+     [turn on many GCC warnings (for developers; best with GNU make)])],
+  [case $enableval in
+     yes|no) ;;
+     *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
+   esac
+   gl_gcc_warnings=$enableval],
+  [
+   # GCC provides fine-grained control over diagnostics which
+   # is used in gnulib for example to suppress warnings from
+   # certain sections of code.  So if this is available and
+   # we're running from a git repo, then auto enable the warnings.
+   gl_gcc_warnings=no
+   gl_GCC_VERSION_IFELSE([4], [6], [test -d "$srcdir"/.git && gl_gcc_warnings=yes])]
+)
+
 if test "$gl_gcc_warnings" = yes; then
   gl_WARN_ADD([-Werror], [WERROR_CFLAGS])
   AC_SUBST([WERROR_CFLAGS])