devel/cdecl: update to 6.6.2.

Added atomic_char8_t
Added atomic_char8_t typedef for C++20.

Fixed flex & bison dependency
Now insists on flex & bison only when lexer.c & parser.c don't exist (i.e., when building from a git clone that doesn't include those files rather than an official release that does).

Explicit structure definition error
Structure declarations (that were never supported) now produce a specific error message:

cdecl> explain struct S { int x; };

^

18: error: structure definitions are not supported
Fixed libreadline & libcurses dependency
Moved check for libcurses before libreadline since some implementations of the latter depend on the former (e.g., Solaris 11).

More gcc warnings
Enabled more gcc warnings and fixed them.

thread-local
Added this (with a -) as an English synonym for thread_local (with an _).