Resync config/ from GCC.

config/weakref.m4

@@ -0,0 +1,47 @@
+
+dnl Check if the target supports weak.
+AC_DEFUN([GCC_CHECK_ATTRIBUTE_WEAK], [
+  AC_CACHE_CHECK([whether the target supports weak],
+		 ac_cv_have_attribute_weak, [
+  weakref_m4_saved_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS -Werror"
+  AC_TRY_COMPILE([void __attribute__((weak)) foo(void) { }],
+		 [], ac_cv_have_attribute_weak=yes,
+		 ac_cv_have_attribute_weak=no)
+  CFLAGS="$weakref_m4_saved_CFLAGS"])
+  if test x"$ac_cv_have_attribute_weak" = xyes; then
+    AC_DEFINE(HAVE_ATTRIBUTE_WEAK, 1,
+      [Define to 1 if the target supports __attribute__((weak)).])
+  fi])
+
+dnl Check whether weak refs work like the ELF ones.
+dnl This means that the weak reference works without having to satify 
+dnl linkage for the item.
+dnl There are targets (at least Darwin) where we have fully functional
+dnl weakrefs at runtime, but must supply the referenced item at link time.
+AC_DEFUN([GCC_CHECK_ELF_STYLE_WEAKREF], [
+  AC_CACHE_CHECK([whether weak refs work like ELF],
+                  ac_cv_have_elf_style_weakref, [
+  weakref_m4_saved_CFLAGS="$CFLAGS"
+  case "${host}" in
+    *-apple-darwin*) CFLAGS="$CFLAGS -Wl,-undefined,dynamic_lookup" ;;
+    *) ;;
+  esac  
+  AC_RUN_IFELSE([AC_LANG_SOURCE([[
+extern void fNotToBeFound(void) __attribute__((weak));
+int main () 
+{
+  if (fNotToBeFound)
+    return 1;
+  else
+    return 0;
+}
+]])], ac_cv_have_elf_style_weakref=yes, ac_cv_have_elf_style_weakref=no, [
+case "${host}" in
+  *-apple-darwin[[89]]*) ac_cv_have_elf_style_weakref=no ;;
+  *) ac_cv_have_elf_style_weakref=yes;;
+esac])CFLAGS="$weakref_m4_saved_CFLAGS"])
+if test x"$ac_cv_have_elf_style_weakref" = xyes; then
+  AC_DEFINE(HAVE_ELF_STYLE_WEAKREF, 1, [Define to 1 if target has a weakref that works like the ELF one.])
+fi])
+