Unbreak build when no system ocaml installed

Summary:
If ocaml is not installed (or if homebrew has sandboxed it out), `opam
init` fails unless passed `--compiler`.

Reviewed By: sblackshear, jvillard

Differential Revision: D3927532

fbshipit-source-id: d2fbec9

build-infer.sh

@@ -98,10 +98,10 @@ check_installed () {
 setup_opam () {
     OCAML_VERSION="4.02.3"
 
-    opam init -j $NCPU --no-setup --yes
+    opam init --compiler=$OCAML_VERSION -j $NCPU --no-setup --yes
 
     OPAMSWITCH=infer-$OCAML_VERSION
-    opam switch install -j $NCPU $OPAMSWITCH --alias-of $OCAML_VERSION || true
+    opam switch set -j $NCPU $OPAMSWITCH --alias-of $OCAML_VERSION
 }
 
 add_opam_git_pin () {