Allows users to set this as an environment variable before running
the setup script, which may be useful on distributions shipping
non-standard packages.
* setup_dxvk.sh: Fix typo
* [general] README: Document required meson version
Meson 0.43 introduced support for @BASENAME@ in the generator argument
list, so document that we need at least that.
* [general] README: Document mingw-w64 threading support requirement