Add missing variable to the README

README.md

@@ -278,7 +278,7 @@ You can override the temporary file containing information
 about Erlang's environment by setting the `C_SRC_ENV` variable.
 This file is automatically generated on first run.
 
-The `CC`, `CFLAGS`, `CXXFLAGS`, `LDLIBS` and `LDFLAGS` variables
+The `CC`, `CXX`, `CFLAGS`, `CXXFLAGS`, `LDLIBS` and `LDFLAGS` variables
 may be modified or replaced with any value of your choosing.
 The defaults are system dependent.