msys problems: do not rely on an extern variable, use api instead

[dk]

Hello,

The msys/win32 problems seems to be still unsolved in 2022 when one relies on exported variables, rather than exported exported functions. There, code trying to reach char* libx265.x265_version fails with dll resolve error. I would guess one could fix that either by nudging libx265 to properly export their symbols on msys/win32, or some other way, but this fix I applied seems the cleanest.

Kindly review Regards, Dmitry