System emulation executables with SDL are typically windows
executables. Sometimes console executables are more useful,
so create both variants if linker option -mwindows was detected.

v2:
This version uses QEMU_PROGW / QEMU_PROG instead of QEMU_PROG / QEMU_PROGC.
Signed-off-by: NStefan Weil <sw@weilnetz.de>