Support MSVC compiler

[jonahbeckford]

1. Define YAML_DECLARE_EXPORT so vendored yaml C code does not use the default "import" which is for when there is already a yaml DLL; without it MSVC complains that the functions are being defined even though DLL import is on. That is:

# writer.c(33): error C2491: 'yaml_emitter_flush': definition of dllimport function not allowed
# writer.c(107): warning C4244: '=': conversion from 'unsigned int' to 'unsigned char', possible loss of data
# writer.c(117): warning C4244: '=': conversion from 'unsigned int' to 'unsigned char', possible loss of data

2. Use .lib and .obj on Windows, which fixes:

# File "types/stubgen/dune", line 11, characters 0-232:
# 11 | (rule
# 12 |  (targets ffi_ml_types_stubgen.exe)
# 13 |  (deps    (:c ./ffi_ml_types_stubgen.c) ../../vendor/libyaml_c_stubs.a)
# 14 |  (action (run %{cc} %{c} -I../../vendor %{read-lines:../../config/ctypes-cflags}
# 15 |   -I %{ocaml_where} -o %{targets})))
# Error: No rule found for vendor/libyaml_c_stubs.a

[avsm]

Thanks! This looks good to merge.