It is a goal to compile perl with g++.
g++-6 adds stricter -fpermissive checks, which fail in Encode 2.87.
Maybe add more core-compatible warnings to Makefile.PL CCFLAGS:
-Wpointer-sign, -fpermissive, -Werror=declaration-after-statement

static const array forward declarations are illegal in C++, they need to be extern.
And we cannot just ask $Config{d_cplusplus} since you are allowed to override it via
perl Makefile.PL CC=g++-6.

Some -Wc++11-compat fixes are also missing.

I'll try to come up with PR, as I need it for cperl.