Bug 1306650 - part 11 - use a relative path for the build directory on windows; r=ehsan

In a taskcluster world, we cannot used fixed directories, since we don't
know the absolute path of the directory we're building in ahead of time.
(We could pass it in to the build script, or discover it in the script
itself, but that wouldn't really solve the next problem.)  This change
does make the builds not reproducible, but as we're using clang-cl
purely for secondary purposes on Windows, rather than for shipping
Firefox binaries (as we would on Mac, say), I don't feel bad about
punting the reproducibility issue down the road a bit.

build/build-clang/build-clang.py

@@ -230,7 +230,15 @@ if __name__ == "__main__":
     # cleans it up automatically.
     base_dir = "/builds/slave/moz-toolchain"
     if is_windows():
-        base_dir = "c:%s" % base_dir
+        # TODO: Because Windows taskcluster builds are run with distinct
+        # user IDs for each job, we can't store things in some globally
+        # accessible directory: one job will run, checkout LLVM to that
+        # directory, and then if another job runs, the new user won't be
+        # able to access the previously-checked out code--or be able to
+        # delete it.  So on Windows, we build in the task-specific home
+        # directory; we will eventually add -fdebug-prefix-map options
+        # to the LLVM build to bring back reproducibility.
+        base_dir = os.path.join(os.getcwd(), 'llvm-sources')
 
     source_dir = base_dir + "/src"
     build_dir = base_dir + "/build"