SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Add external include paths to ExternalIncludePath instead of
AdditionalIncludeDirectories. msbuild appends these paths to
EXTERNAL_INCLUDE env var, which is passed to /external:env:.
Specify /external:W0 and /external:templates-, with override for
DolphinQt for the template flag, since Qt 5.15.0 causes some warnings
in qmap.h
Intellisense doesn't like defines in PCH files, and it doesn't like the deleted
constructor for BitField. (I think it's being overly strict about the
"must have no non-default constructors" rule for classes in unions.)
