sigrok-cross-mingw: Optionally emit progress messages.

sigrok-cross-mingw: Optionally emit progress messages.

The cross compile script executes a lot of steps, some of them rather
expensive and complex, before first output from child processes appears
on screen (libserialport clone being the first typical output). Prepare
to optionally emit progress messages for developers' awareness. Just set
the $ECHO environment variable to suppress these.