attempt to specify boost to build with clang - take 2

Summary:
Original commit changeset: c6f013565680

Redo D22417488 (https://github.com/facebook/folly/commit/43d80f110b181dd7f72f0f935322a58f9cbb5147), with D22577887 fixing open source CI builds

Reviewed By: wez

Differential Revision: D22577884

fbshipit-source-id: 0c94483e05715493859d29bc2b58f0847a95826b

build/fbcode_builder/getdeps/builder.py

@@ -802,8 +802,16 @@ class Boost(BuilderBase):
         linkage = ["static"]
         if self.build_opts.is_windows():
             linkage.append("shared")
+
+        args = []
+        if self.build_opts.is_darwin():
+            clang = subprocess.check_output(["xcrun", "--find", "clang"])
+            user_config = os.path.join(self.build_dir, "project-config.jam")
+            with open(user_config, "w") as jamfile:
+                jamfile.write("using clang : : %s ;\n" % clang.decode().strip())
+            args.append("--user-config=%s" % user_config)
+
         for link in linkage:
-            args = []
             if self.build_opts.is_windows():
                 bootstrap = os.path.join(self.src_dir, "bootstrap.bat")
                 self._run_cmd([bootstrap], cwd=self.src_dir)

build/fbcode_builder/manifests/boost

@@ -81,3 +81,6 @@ builder = boost
 --with-timer
 --with-type_erasure
 --with-wave
+
+[b2.args.os=darwin]
+toolset=clang