makefiles/vars.inc.mk: export CXXINCLUDES

It is used by Makefile.base so should be exported globally even if only
defined by `llvm.inc.mk` for the moment.

makefiles/vars.inc.mk

@@ -12,6 +12,7 @@ export CPU                   # The CPU, set by the board's Makefile.include.
 export CPU_MODEL             # The specific identifier of the used CPU, used for some CPU implementations to differentiate between different memory layouts
 export MCU                   # The MCU, set by the board's Makefile.include, or defaulted to the same value as CPU.
 export INCLUDES              # The extra include paths, set by the various Makefile.include files.
+export CXXINCLUDES           # The extra include paths for c++, set by the various Makefile.include files.
 
 export USEMODULE             # Sys Module dependencies of the application. Set in the application's Makefile.
 export USEPKG                # Pkg dependencies (third party modules) of the application. Set in the application's Makefile.