Mplay not cross-compilable to windows and fails on musl

[bernardassan]

Currently, mplay can't be compiled with mingw and musl libc #3 (comment)

• musl libc stdin and stdout are declared as const while mplay.c tries to modify them which leds to a compile error on musl. Look into using freopen
• mplay uses 'sys/wait.h' which is a posix system header and not available on mingw