Build support for OCaml MSVC port

[modlfo]

Hi, I'm using the ollvm library in different platforms. So far for the only one that have had problems when building is in Windows with the MSVC port. If fails because the build system assumes that the extension of the native libraries is ".a" while with MSVC port can be ".lib".

Here's the displayed error

### stderr ###
# Installed C:\cygwin\home\appveyor\.opam\4.03.0+msvc64\lib\ollvm\ollvm_lexer.cmi
# Installed C:\cygwin\home\appveyor\.opam\4.03.0+msvc64\lib\ollvm\ollvm_printer.cmi
# Installed C:\cygwin\home\appveyor\.opam\4.03.0+msvc64\lib\ollvm\ollvm_ez.cmi
# Installed C:\cygwin\home\appveyor\.opam\4.03.0+msvc64\lib\ollvm\ollvm_ast.cmi
# Installed C:\cygwin\home\appveyor\.opam\4.03.0+msvc64\lib\ollvm\ollvm.cmi
# ocamlfind: ollvm.a: No such file or directory
# make[1]: *** [../Makefile.common:91: install] Error 2
# make: *** [Makefile:584: install] Error 2