Requested agent syntax is "Dolphin/5.0-23456"

Fixes a crash that could occur if the static constructor function for
the MainSettings.cpp TU happened to run before the variables in
Common/Version.cpp are initialised. (This is known as the static
initialisation order fiasco.)

By using wrapper functions, those variables are now guaranteed to be
constructed on first use.

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.

Any file which includes scmrev.h must be rebuilt when scmrev.h
is regenerated. By not including scmrev.h from any file other
than Version.cpp, incremental builds become a little faster.

Ideally Common.h wouldn't be a header in the Common library, and instead be renamed to something else, like PlatformCompatibility.h or something, but even then, there's still some things in the header that don't really fall under that label

This moves the version strings out to their own version header that doesn't dump a bunch of other unrelated things into scope, like what Common.h was doing.

This also places them into the Common namespace, as opposed to letting them sit in the global namespace.

OS X uses a case insensitive filesystem by default: when I try to build,
a system header does #include <assert.h>, which picks up
Source/Core/Common/Assert.h.  This only happens because CMakeLists adds
'${PROJECT_BINARY_DIR}/Source/Core/Common' as an include directory: in
an out-of-tree build, that directory contains no other source files, but
in an in-tree build PROJECT_BINARY_DIR is just the source root.

This is only used for scmrev.h.  Change the include directory to
'${PROJECT_BINARY_DIR}/Source/Core' and the include to
"Common/scmrev.h", which is more consistent with normal headers anyway.

Used to distinguish between Dolphin distributions (e.g. "dolphin-emu.org",
"Ishiiruka", etc.).

Those macros may be defined, or not. We should support both cases, so use std::string as it also defines the length of the string.