• Fix build after SDL2_Mixer update

Don't try to check for SDL2_Mixer at configure phase by trying to
run program which tries Mix_LoadMUS.

First, the program is incorrect as per SDL2_Mixer documentation,
one is not allowed to call any SDL2_Mixer functions without first
calling Mix_Init() (which is not called).

Next, calling any mixer functions may involve opening audio device,
which may not be present on the machine where the package is built.

Fix by forcing the broken check to pass.

PR: 235557
Reported by: jbeich@FreeBSD.org
Approved by: portmgr blanket (build fix)
Upstream issue: https://github.com/sekaiproject/ponscripter-fork/issues/92