define option to disable cpp allocation overrides (#1145)

Setting `PICO_CXX_DISABLE_ALLOCATION_OVERRIDES=1` will prevent `new` etc. operators being overridden

src/rp2_common/pico_standard_link/new_delete.cpp

@@ -4,9 +4,10 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#if !PICO_CXX_ENABLE_EXCEPTIONS
+#if !PICO_CXX_ENABLE_EXCEPTIONS 
 // Override the standard allocators to use regular malloc/free
 
+#if !PICO_CXX_DISABLE_ALLOCATION_OVERRIDES // Let user override
 #include <cstdlib>
 
 void *operator new(std::size_t n) {
@@ -30,3 +31,5 @@ void operator delete[](void *p, __unused std::size_t n) noexcept { std::free(p);
 #endif
 
 #endif
+
+#endif