bug#75939: bug in compile wrapper when using MSVC from Msys2

Hi Kirill - thanks for the report and proposed fix.

    export MSYS2_ARG_CONV_EXCL='-Tp'

Setting an environment variable seems fairly safe, since if an older
version of the compiler doesn't pay attention to it, at least the
problem will not become worse.

However, I'd like to hear from others who use msys, cl, Windows, etc.
(Since I don't), as to whether setting this in the compile script would
cause any problems. Anyone? --thanks, karl.