Fix for MSVC port: do not warn for use of deprecated declarations.

[jhjourdan]

No description provided.

[jhjourdan]

Unison currently patches lablgtk at each Windows build, so merging this would really help Unison developers. See #165 .

[MisterDA]

FYI there's a cool way to specify warnings for both cc-like and MSVC-like compilers:

(rule
 (with-stdout-to cflags.cc (echo "(-Wno-deprecated-declarations)")))

(rule
 (with-stdout-to cflags.msvc (echo "")))

(library
 (name mylib)
 (foreign_stubs
  (language c)
  (flags :standard (:include cflags.%{ocaml-config:ccomp_type}))
  (names mystubs)))