Ensure that C++ modules can be built against a perl configured with -DPERL_GLOBAL_STR...

Ensure that C++ modules can be built against a perl configured with -DPERL_GLOBAL_STRUCT_PRIVATE.

Perl_GetVarsPrivate() could not be seen from within a module built
with a C++ compiler if the perl binary was built with a C compiler,
as the prototype was not 'extern "C"' for C++.
The change moves the EXTERN_C defines so they can be used for the
prototype of Perl_GetVarsPrivate().