Fix 'replacement operator delete cannot be inline' GCC warning (Cygwin)

Fix 'replacement operator delete cannot be inline' GCC warning (Cygwin)
(fix commits 83e0a2c, 3379238)

This commit also eliminates
"operator delete is missing exception specification throw()" compiler
warning.

* gc_cpp.cc (operator delete): Define unconditionally (i.e. for Cygwin
too).
* include/gc_cpp.h [__CYGWIN__]: Do not include "new" header.
* include/gc_cpp.h [__CYGWIN__] (operator delete): Remove (as
replacement function 'operator delete' cannot be declared 'inline').