Enable parallel compilation on Windows builds

[jkloth]

BPO	46778
Nosy	@pfmoore, @tjguk, @jkloth, @zware, @zooba
PRs	bpo-46778: Enable multiprocess compilation for source files #31390

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-02-17.18:41:07.598>
created_at = <Date 2022-02-17.16:24:10.526>
labels = ['build', 'OS-windows', '3.11']
title = 'Enable parallel compilation on Windows builds'
updated_at = <Date 2022-02-17.18:41:09.287>
user = 'https://github.com/jkloth'

bugs.python.org fields:

activity = <Date 2022-02-17.18:41:09.287>
actor = 'steve.dower'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-17.18:41:07.598>
closer = 'steve.dower'
components = ['Build', 'Windows']
creation = <Date 2022-02-17.16:24:10.526>
creator = 'jkloth'
dependencies = []
files = []
hgrepos = []
issue_num = 46778
keywords = ['patch']
message_count = 3.0
messages = ['413412', '413431', '413432']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'jkloth', 'zach.ware', 'steve.dower']
pr_nums = ['31390']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46778'
versions = ['Python 3.11']

[jkloth]

While the current build does enable building of projects in parallel
(msbuild -m), the compilation of each project's source files is done sequentially. For large projects like pythoncore or _freeze_module this can take quite some time.

This simple PR speeds things up significantly, ~2x on machines that I have access.

[zooba]

Easy, and thanks for the PR!

[zooba]

New changeset 98dd0ae by Jeremy Kloth in branch 'main':
bpo-46778: Enable multiprocess compilation for source files when building on Windows (GH-31390)
98dd0ae