Arrange to supply declarations for strtoll/strtoull if needed.

Buildfarm member dromedary is still unhappy about the recently-added
ecpg "long long" tests.  The reason turns out to be that it includes
"-ansi" in its CFLAGS, and in their infinite wisdom Apple have decided
to hide the declarations of strtoll/strtoull in C89-compliant builds.
(I find it pretty curious that they hide those function declarations
when you can nonetheless declare a "long long" variable, but anyway
that is their behavior, both on dromedary's obsolete macOS version and
the newest and shiniest.)  As a result, gcc assumes these functions
return "int", leading naturally to wrong results.

(Looking at dromedary's past build results, it's evident that this
problem also breaks pg_strtouint64() on 32-bit platforms; but we
evidently have no regression tests that exercise that function with
values above 32 bits.)

To fix, supply declarations for these functions when the platform
provides the functions but not the declarations, using the same type
of mechanism as we use for some other similar cases.

Discussion: https://postgr.es/m/151935568942.1461.14623890240535309745@wrigleys.postgresql.org

configure

@@ -23423,6 +23423,143 @@ _ACEOF
 fi
 done
 
+# strto[u]ll may exist but not be declared
+{ $as_echo "$as_me:$LINENO: checking whether strtoll is declared" >&5
+$as_echo_n "checking whether strtoll is declared... " >&6; }
+if test "${ac_cv_have_decl_strtoll+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+$ac_includes_default
+int
+main ()
+{
+#ifndef strtoll
+  (void) strtoll;
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  ac_cv_have_decl_strtoll=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	ac_cv_have_decl_strtoll=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $ac_cv_have_decl_strtoll" >&5
+$as_echo "$ac_cv_have_decl_strtoll" >&6; }
+if test "x$ac_cv_have_decl_strtoll" = x""yes; then
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_STRTOLL 1
+_ACEOF
+
+
+else
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_STRTOLL 0
+_ACEOF
+
+
+fi
+{ $as_echo "$as_me:$LINENO: checking whether strtoull is declared" >&5
+$as_echo_n "checking whether strtoull is declared... " >&6; }
+if test "${ac_cv_have_decl_strtoull+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+$ac_includes_default
+int
+main ()
+{
+#ifndef strtoull
+  (void) strtoull;
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  ac_cv_have_decl_strtoull=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	ac_cv_have_decl_strtoull=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $ac_cv_have_decl_strtoull" >&5
+$as_echo "$ac_cv_have_decl_strtoull" >&6; }
+if test "x$ac_cv_have_decl_strtoull" = x""yes; then
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_STRTOULL 1
+_ACEOF
+
+
+else
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_STRTOULL 0
+_ACEOF
+
+
+fi
+
+
 
 { $as_echo "$as_me:$LINENO: checking for builtin locking functions" >&5
 $as_echo_n "checking for builtin locking functions... " >&6; }