Bug 1563797 - Use 'backports.shutil_which' instead of 'which' in the build system r=firefox-build-system-reviewers,chmanchester

Credit: Callek for figuring out an issue in 'make check' making the binary
absolute in mozbuild.base.

Differential Revision: https://phabricator.services.mozilla.com/D37319

build/build-clang/build-clang.py

@@ -14,11 +14,12 @@ import fnmatch
 import glob
 import errno
 import re
-from contextlib import contextmanager
 import sys
-import which
+from contextlib import contextmanager
 from distutils.dir_util import copy_tree
 
+from mozfile import which
+
 
 def symlink(source, link_name):
     os_symlink = getattr(os, "symlink", None)
@@ -436,10 +437,10 @@ def get_tool(config, key):
             return f
 
     # Assume that we have the name of some program that should be on PATH.
-    try:
-        return which.which(f) if f else which.which(key)
-    except which.WhichError:
-        raise ValueError("%s not found on PATH" % f)
+    tool = which(f) if f else which(key)
+    if not tool:
+        raise ValueError("%s not found on PATH" % (f or key))
+    return tool
 
 
 # This function is intended to be called on the final build directory when