AppVeyor, Travis: treat OCaml warnings as errors

OCaml warnings still treated as only warnings during regular builds
outside the build bots.

.travis.yml

@@ -21,6 +21,10 @@ matrix:
     - os: osx
       env: COMPILER=4.03 PACKAGE_MANAGER=macports LIBEV=yes
 
+env:
+  global:
+    - LWT_WARNINGS_AS_ERRORS=yes
+
 script: bash -e utils/travis.sh
 
 notifications:

appveyor.yml

@@ -7,6 +7,9 @@ environment:
     - {SYSTEM: cygwin, ARCH: x86, COMPILER: 4.04}
     - {SYSTEM: mingw, ARCH: x86_64, COMPILER: 4.03}
 
+  global:
+    LWT_WARNINGS_AS_ERRORS: yes
+
 install:
   - 'IF "%ARCH%"=="x86" ( SET CYGSH=C:\Cygwin\bin\bash -lc ) ELSE ( SET CYGSH=C:\Cygwin64\bin\bash -lc )'
   - ps: '(new-object net.webclient).DownloadString("https://aantron.github.io/binaries/$env:SYSTEM/$env:ARCH/ocaml/$env:COMPILER/install.ps1") | PowerShell -Command -'

myocamlbuild.ml

@@ -65,6 +65,16 @@ let define_c_library name env =
     flag ["link"; "ocaml"; tag] & S (List.map (fun arg -> S[A"-cclib"; arg]) lib)
   end
 
+let conditional_warnings_as_errors () =
+  match Sys.getenv "LWT_WARNINGS_AS_ERRORS" with
+  | "yes" ->
+    let flags = S [A "-warn-error"; A "+A"] in
+    flag ["ocaml"; "compile"] flags;
+    flag ["ocaml"; "link"] flags
+
+  | _ -> ()
+  | exception Not_found -> ()
+
 let () =
   dispatch
     (fun hook ->
@@ -119,7 +129,9 @@ let () =
              end;
 
              List.iter (fun name ->
-               mark_tag_used (c_library_tag name)) c_libraries
+               mark_tag_used (c_library_tag name)) c_libraries;
+
+             conditional_warnings_as_errors ()
 
          | _ ->
              ())