These messages can be localized, so we can't just assume it's all ASCII.

Gets these functions out of the global namespace.

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.

I don't know why Android does it like this, but at least
it's easy to fix. (We are currently at __ANDROID_API__ == 21.)

This unifies GetModuleFileName calls between Dolphin and WinUpdater
and allows to gracefully remove MAX_PATH limit from GetExePath

Trying to force the XSI version by undefining _GNU_SOURCE can lead
to compilation errors on some systems because of headers expecting
that _GNU_SOURCE is defined.

This commit uses define checks to detect which version we have.
I tried making an overloaded function (int and const char*) instead,
but that led to a warning about one of the variants being unused.