Allow -l option to ocamlmklib on MSVC ports

In general (especially for the Windows API), -lfoo cannot be used to
link libraries because the various drivers assume that libfoo.lib is
meant. This patch allows ocamlmklib to mask this difference so that
-luser32 will refer to libuser32.a as normal on MinGW but user32.lib on
MSVC.

Changes

@@ -82,6 +82,9 @@ Working version
   attribute is hidden by signature coercion
   (Alain Frisch, report by bmillwood, review by Leo White)
 
+- GPR#1189: allow MSVC ports to use -l option in ocamlmklib
+  (David Allsopp)
+
 ### Manual and documentation:
 
 - PR#6548: remove obsolete limitation in the description of private

tools/ocamlmklib.ml

@@ -101,6 +101,12 @@ let parse_arguments argv =
     else if s = "-linkall" then
       caml_opts := s :: !caml_opts
     else if starts_with s "-l" then
+      let s =
+        if Config.ccomp_type = "msvc" then
+          String.sub s 2 (String.length s - 2) ^ ".lib"
+        else
+          s
+      in
       c_libs := s :: !c_libs
     else if starts_with s "-L" then
      (c_Lopts := s :: !c_Lopts;