wgl: Support driconf

This series hooks up the driconf infrastructure to the wgl frontend. In order to do that, I also:

• Refactored the parsing of driconf -> st options into something that can be shared outside of the dri frontend.
• A build fix when xmlconfig is linked into the MinGW CI build, which is debug (-O0). A non-static inline function body is only actually emitted by GCC during optimization passes, so running -O0 ends up never emitting the body, producing linker errors.

Windows doesn't have expat, so it'll only use the static driconf, but this also enables the getenv environment overrides.

/cc @robclark @anholt since we were discussing this a bit in !12135 (merged)

Closes: #4493 (closed)