libs/libxx/Kconfig: CONFIG_LIBC_LOCALE is needed to build libcxx. A

math library is also needed, however, here are various ways to provide a
math library so CONFIG_LIBM is not now set.

boards/arm/stm32l4/nucleo-l476rg/scripts/Make.defs:  Add required
definitiions if libcxx is enabled.

boards/arm/stm32l4/nucleo-l476rg/scripts/Make.defs

@@ -47,12 +47,20 @@ ifeq ($(WINTOOL),y)
   ARCHINCLUDES = -I. -isystem "${shell cygpath -w $(TOPDIR)/include}"
   ARCHXXINCLUDES = -I. -isystem "${shell cygpath -w $(TOPDIR)/include}" -isystem "${shell cygpath -w $(TOPDIR)/include/cxx}"
   ARCHSCRIPT = -T "${shell cygpath -w $(TOPDIR)/boards/$(CONFIG_ARCH)/$(CONFIG_ARCH_CHIP)/$(CONFIG_ARCH_BOARD)/scripts/$(LDSCRIPT)}"
+
+  ifeq ($(CONFIG_LIBCXX),y)
+    ARCHXXINCLUDES += -isystem "${shell cygpath -w $(TOPDIR)/include/libcxx}"
+  endif
 else
   # Linux/Cygwin-native toolchain
   MKDEP = $(TOPDIR)/tools/mkdeps$(HOSTEXEEXT)
   ARCHINCLUDES = -I. -isystem $(TOPDIR)/include
   ARCHXXINCLUDES = -I. -isystem $(TOPDIR)/include -isystem $(TOPDIR)/include/cxx
   ARCHSCRIPT = -T$(TOPDIR)/boards/$(CONFIG_ARCH)/$(CONFIG_ARCH_CHIP)/$(CONFIG_ARCH_BOARD)/scripts/$(LDSCRIPT)
+
+  ifeq ($(CONFIG_LIBCXX),y)
+    ARCHXXINCLUDES += -isystem $(TOPDIR)/include/libcxx
+  endif
 endif
 
 CC = $(CROSSDEV)gcc
@@ -90,6 +98,21 @@ CXXPICFLAGS = $(ARCHPICFLAGS) $(CXXFLAGS)
 CPPFLAGS = $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRADEFINES)
 AFLAGS = $(CFLAGS) -D__ASSEMBLY__
 
+ifeq ($(CONFIG_LIBCXX),y)
+  CXXFLAGS += -std=c++14 -D__NuttX__ -D__GLIBCXX__ -DMB_LEN_MAX=8 -DHAVE_DEPENDENT_EH_ABI=0 \
+    -fno-builtin -D_DEBUG -D_LIBCPP_BUILD_STATIC -D_LIBCPP_STD_VER=14
+
+  ifneq ($(CONFIG_LIBCXX_EXCEPTION),y)
+    CXXFLAGS += -fno-exceptions -fno-rtti -fcheck-new -D_LIBCPP_NO_EXCEPTIONS
+  endif
+
+  # supc++ is always needed with libcxx
+
+  LIBSUPXX = ${shell $(CC) $(CFLAGS) --print-file-name=libsupc++.a}
+  EXTRA_LIBPATHS = -L "${shell dirname "$(LIBSUPXX)"}"
+  EXTRA_LIBS = -lsupc++
+endif
+
 NXFLATLDFLAGS1 = -r -d -warn-common
 NXFLATLDFLAGS2 = $(NXFLATLDFLAGS1) -T$(TOPDIR)/binfmt/libnxflat/gnu-nxflat-pcrel.ld -no-check-sections
 LDNXFLATFLAGS = -e main -s 2048

libs/libxx/Kconfig

@@ -43,6 +43,7 @@ config LIBCXX
 	default n
 	select CLOCK_MONOTONIC
 	select LIBC_WCHAR
+	select LIBC_LOCALE
 	select TIME_EXTENDED
 	---help---
 		If you have installed libcxx into the NuttX source tree, then it can