Suggested by Jake Page <jake at cs dot stanford dot edu>
in http://sources.redhat.com/ml/crossgcc/2004-07/msg00169.html
but the libc-symbols.h fragment has been reworked to fix typos
and to still work on gcc-2.95.3, which lacks _Pragma.

His comment was:
 Fixes gcc-3.4 optimizing out comparisons to weak external symbols in code
 like:
  if (__pthread_mutex_init != NULL)
    __pthread_mutex_init(...);
 that causes segfaults during libc init when not linked to libpthread

diff -uNr glibc-2.2.5-orig/resolv/res_libc.c glibc-2.2.5/resolv/res_libc.c
--- glibc-2.2.5-orig/resolv/res_libc.c	2001-02-12 12:23:34.000000000 -0800
+++ glibc-2.2.5/resolv/res_libc.c	2004-07-27 22:56:32.000000000 -0700
@@ -84,7 +84,8 @@
 
 #if SHLIB_COMPAT(libc, GLIBC_2_0, GLIBC_2_2)
 # undef res_init
-strong_alias (__res_init, __res_init_weak);
+extern int __res_init_weak (void);
 weak_extern (__res_init_weak);
+strong_alias (__res_init, __res_init_weak);
 compat_symbol (libc, __res_init_weak, res_init, GLIBC_2_0);
 #endif
--- glibc-2.2.5/include/libc-symbols.h.old	Fri Aug 13 04:20:55 2004
+++ glibc-2.2.5/include/libc-symbols.h	Fri Aug 13 04:37:06 2004
@@ -108,10 +108,16 @@
 
 /* Declare SYMBOL as weak undefined symbol (resolved to 0 if not defined).  */
 #  define weak_extern(symbol) _weak_extern (symbol)
-#  ifdef HAVE_ASM_WEAKEXT_DIRECTIVE
-#   define _weak_extern(symbol) asm (".weakext " __SYMBOL_PREFIX #symbol);
-#  else
-#   define _weak_extern(symbol)    asm (".weak " __SYMBOL_PREFIX #symbol);
+#  if __GNUC__ >= 3
+    /* see http://sources.redhat.com/ml/libc-alpha/2003-01/msg00043.html */
+#   define __weak_extern_1(expr) _Pragma(#expr)
+#   define _weak_extern(symbol) __weak_extern_1(weak symbol)
+#  else /* __GNUC__ == 2 */
+#   ifdef HAVE_ASM_WEAKEXT_DIRECTIVE
+#    define _weak_extern(symbol) asm (".weakext " __SYMBOL_PREFIX #symbol);
+#   else
+#    define _weak_extern(symbol)    asm (".weak " __SYMBOL_PREFIX #symbol);
+#   endif
 #  endif
 
 # else