Do not include bits/functexcept.h when building with libc++

This header exists only in GCC's libstdc++. If we are building on a
system using exclusively LLVM's libc++, compilation will fail.

Makefile

@@ -44,7 +44,11 @@ OBJECTS := $(SOURCES:.c=.o)
 ifeq ($(CONFIG_CXX_ALLOCATOR),true)
     # make sure LTO is compatible in case CC and CXX don't match (such as clang and g++)
     CXX := $(CC)
-    LDLIBS += -lstdc++
+    ifeq ($(CONFIG_USE_LIBCXX),true)
+        LDLIBS += -lc++
+    else
+        LDLIBS += -lstdc++
+    endif
 
     SOURCES += new.cc
     OBJECTS += new.o
@@ -85,6 +89,7 @@ ifeq (,$(filter $(CONFIG_STATS),true false))
 endif
 
 CPPFLAGS += \
+    -DCONFIG_USE_LIBCXX=$(CONFIG_USE_LIBCXX) \
     -DCONFIG_SEAL_METADATA=$(CONFIG_SEAL_METADATA) \
     -DZERO_ON_FREE=$(CONFIG_ZERO_ON_FREE) \
     -DWRITE_AFTER_FREE_CHECK=$(CONFIG_WRITE_AFTER_FREE_CHECK) \

config/default.mk

@@ -1,6 +1,7 @@
 CONFIG_WERROR := true
 CONFIG_NATIVE := true
 CONFIG_CXX_ALLOCATOR := true
+CONFIG_USE_LIBCXX := false
 CONFIG_UBSAN := false
 CONFIG_SEAL_METADATA := false
 CONFIG_ZERO_ON_FREE := true

config/light.mk

@@ -1,6 +1,7 @@
 CONFIG_WERROR := true
 CONFIG_NATIVE := true
 CONFIG_CXX_ALLOCATOR := true
+CONFIG_USE_LIBCXX := false
 CONFIG_UBSAN := false
 CONFIG_SEAL_METADATA := false
 CONFIG_ZERO_ON_FREE := true

new.cc

@@ -1,4 +1,6 @@
+#if !defined(__clang__) || !CONFIG_USE_LIBCXX
 #include <bits/functexcept.h>
+#endif
 #include <new>
 
 #include "h_malloc.h"