Replace experimental_infallible_new with compile-time flag

The runtime option aborted on every OOM, breaking new(std::nothrow)
semantics. Replace with configure-time --enable-cxx-infallible-new
(default off): when on, throwing new aborts (size logged) and
nothrow returns null; when off, standard new_handler + bad_alloc /
null behavior is preserved. Under LTO the on-path lets the compiler
prove operator new is no-throw.

test/integration/cpp/failing_new.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+XMALLOC_STR=""
+if [ "x${enable_xmalloc}" = "x1" ] ; then
+  XMALLOC_STR="xmalloc:false"
+fi
+
+export MALLOC_CONF="${XMALLOC_STR}"

test/integration/cpp/infallible_new.cpp

@@ -3,8 +3,9 @@
 #include "test/jemalloc_test.h"
 
 /*
- * We can't test C++ in unit tests.  In order to intercept abort, use the
- * internal test hook in integration tests.
+ * Verifies that, when jemalloc is built with --enable-cxx-infallible-new,
+ * throwing operator new on OOM aborts via safety_check_fail. The test hook
+ * intercepts the abort and asserts the size-bearing message prefix.
  */
 bool fake_abort_called;
 void

test/integration/cpp/infallible_new.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+XMALLOC_STR=""
+if [ "x${enable_xmalloc}" = "x1" ] ; then
+  XMALLOC_STR="xmalloc:false"
+fi
+
+export MALLOC_CONF="${XMALLOC_STR}"

test/integration/cpp/infallible_new_false.sh

@@ -1,8 +0,0 @@
-#!/bin/sh
-
-XMALLOC_STR=""
-if [ "x${enable_xmalloc}" = "x1" ] ; then
-  XMALLOC_STR="xmalloc:false,"
-fi
-
-export MALLOC_CONF="${XMALLOC_STR}experimental_infallible_new:false"

test/integration/cpp/infallible_new_true.sh

@@ -1,8 +0,0 @@
-#!/bin/sh
-
-XMALLOC_STR=""
-if [ "x${enable_xmalloc}" = "x1" ] ; then
-  XMALLOC_STR="xmalloc:false,"
-fi
-
-export MALLOC_CONF="${XMALLOC_STR}experimental_infallible_new:true"