safely check for standalone interpreter sentinel files (#750)

python/pip_install/pip_repository.bzl

diff --git a/python/pip_install/pip_repository.bzl b/python/pip_install/pip_repository.bzl
index 8b7479a..837d459 100644
--- a/python/pip_install/pip_repository.bzl
+++ b/python/pip_install/pip_repository.bzl
@@ -73,10 +73,12 @@
     Pip won't be able to compile c extensions from sdists with the pre built python distributions from indygreg
     otherwise. See https://github.com/indygreg/python-build-standalone/issues/103
     """
+    python_interpreter_workspace = rctx.path(Label("@{}//:WORKSPACE".format(rctx.attr.python_interpreter_target.workspace_name))).dirname
     if (
         rctx.os.name.lower().startswith("mac os") and
         rctx.attr.python_interpreter_target != None and
-        rctx.path(Label("@{}//:{}".format(rctx.attr.python_interpreter_target.workspace_name, STANDALONE_INTERPRETER_FILENAME))) and
+        # This is a rules_python provided toolchain.
+        rctx.execute(["ls", "{}/{}".format(python_interpreter_workspace, STANDALONE_INTERPRETER_FILENAME)]).return_code == 0 and
         not environment.get(CPPFLAGS)
     ):
         xcode_sdk_location = rctx.execute(["xcode-select", "--print-path"])