Fix configure's AC_CHECK_DECLS tests to work correctly with clang.

The test case that Autoconf uses to discover whether a function has
been declared doesn't work reliably with clang, because clang reports
a warning not an error if the name is a known built-in function.
On some platforms, this results in a lot of compile-time warnings about
strlcpy and related functions not having been declared.

There is a fix for this (by Noah Misch) in the upstream Autoconf sources,
but since they've not made a release in years and show no indication of
doing so anytime soon, let's just absorb their fix directly.  We can
revert this when and if we update to a newer Autoconf release.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/26819.1542515567@sss.pgh.pa.us

1 files changed, 97 insertions, 3 deletions

diff --git a/configure b/configure
index b15cedcd1bd..c094eb8a142 100755
--- a/configure
+++ b/configure
@@ -2282,13 +2282,16 @@ rm -f conftest.val
 ac_fn_c_check_decl ()
 {
   as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack
-  as_decl_name=`echo $2|sed 's/ *(.*//'`
+  # Initialize each $ac_[]_AC_LANG_ABBREV[]_decl_warn_flag once.
+      as_decl_name=`echo $2|sed 's/ *(.*//'`
   as_decl_use=`echo $2|sed -e 's/(/((/' -e 's/)/) 0&/' -e 's/,/) 0& (/g'`
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $as_decl_name is declared" >&5
 $as_echo_n "checking whether $as_decl_name is declared... " >&6; }
 if eval \${$3+:} false; then :
   $as_echo_n "(cached) " >&6
 else
+  ac_save_werror_flag=$ac_c_werror_flag
+  ac_c_werror_flag="$ac_c_decl_warn_flag$ac_c_werror_flag"
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 $4
@@ -2313,6 +2316,7 @@ else
   eval "$3=no"
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  ac_c_werror_flag=$ac_save_werror_flag
 fi
 eval ac_res=\$$3
 	       { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5
@@ -11902,7 +11906,96 @@ esac
 # posix_fadvise() is a no-op on Solaris, so don't incur function overhead
 # by calling it, 2009-04-02
 # http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/lib/libc/port/gen/posix_fadvise.c
-if test "$PORTNAME" != "solaris"; then
+# The Clang compiler raises a warning for an undeclared identifier that matches
+# a compiler builtin function.  All extant Clang versions are affected, as of
+# Clang 3.6.0.  Test a builtin known to every version.  This problem affects the
+# C and Objective C languages, but Clang does report an error under C++ and
+# Objective C++.
+#
+# Passing -fno-builtin to the compiler would suppress this problem.  That
+# strategy would have the advantage of being insensitive to stray warnings, but
+# it would make tests less realistic.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking how $CC reports undeclared, standard C functions" >&5
+$as_echo_n "checking how $CC reports undeclared, standard C functions... " >&6; }
+if ${ac_cv_c_decl_report+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+(void) strchr;
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -s conftest.err; then :
+      # For AC_CHECK_DECL to react to warnings, the compiler must be silent on
+    # valid AC_CHECK_DECL input.  No library function is consistently available
+    # on freestanding implementations, so test against a dummy declaration.
+    # Include always-available headers on the off chance that they somehow
+    # elicit warnings.
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <float.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <stddef.h>
+extern void ac_decl (int, char *);
+int
+main ()
+{
+#ifdef __cplusplus
+  (void) ac_decl ((int) 0, (char *) 0);
+  (void) ac_decl;
+#else
+  (void) ac_decl;
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -s conftest.err; then :
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "cannot detect from compiler exit status or warnings
+See \`config.log' for more details" "$LINENO" 5; }
+else
+  ac_cv_c_decl_report=warning
+fi
+else
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "cannot compile a simple declaration test
+See \`config.log' for more details" "$LINENO" 5; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+else
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "compiler does not report undeclared identifiers
+See \`config.log' for more details" "$LINENO" 5; }
+fi
+else
+  ac_cv_c_decl_report=error
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_c_decl_report" >&5
+$as_echo "$ac_cv_c_decl_report" >&6; }
+
+case $ac_cv_c_decl_report in
+  warning) ac_c_decl_warn_flag=yes ;;
+  *) ac_c_decl_warn_flag= ;;
+esac
+
+if test "$PORTNAME" != "solaris"; then :
+
 for ac_func in posix_fadvise
 do :
   ac_fn_c_check_func "$LINENO" "posix_fadvise" "ac_cv_func_posix_fadvise"
@@ -11926,7 +12019,8 @@ cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_POSIX_FADVISE $ac_have_decl
 _ACEOF
 
-fi
+
+fi # fi
 
 ac_fn_c_check_decl "$LINENO" "fdatasync" "ac_cv_have_decl_fdatasync" "#include <unistd.h>
 "