mingw: Fix build issues on mingw

At least in the mingw I'm using, there's no cc, but rather gcc, so default to
gcc to avoid having to type CC=gcc all the time. Also append to CFLAGS instead
of overriding them just in case some extra ones are supplied from the command
line.

Finally, the real build issue was DEVICE_TYPE not being defined early enough, so
define it a bit earlier to get it to build in case it wasn't previously defined
in a header.