setup: build extensions in parallel by default

The build_ext distutils command in Python 3.5+ has a "parallel"
option that controls whether to build extensions in parallel. It
is disabled by default (None) and can be set to an integer value
for number of cores or True to indicate use all available CPU
cores.

This commit changes our build_ext command override to set
"parallel" to True unless a value has been provided by the caller.

On my machine, this makes `python setup.py build_ext` 1-4s faster.
It is worth noting that at this time, each individual source file
constituting the extension is still built serially. For Mercurial,
this means that we can't build faster than the slowest-to-build
extension, which is the zstd extension by a long shot. This means
that setup.py is still not very efficient at utilizing multiple
cores. But we're better than before.

Differential Revision: https://phab.mercurial-scm.org/D6923

# no-check-commit because of foo_bar naming

setup.py

@@ -490,6 +490,14 b' class hgbuildext(build_ext):'
 
         return build_ext.initialize_options(self)
 
+    def finalize_options(self):
+        # Unless overridden by the end user, build extensions in parallel.
+        # Only influences behavior on Python 3.5+.
+        if getattr(self, 'parallel', None) is None:
+            self.parallel = True
+
+        return build_ext.finalize_options(self)
+
     def build_extensions(self):
         ruststandalones = [e for e in self.extensions
                            if isinstance(e, RustStandaloneExtension)]