Watcom: Enable optimization

We couldn't enable optimization before the commit 70bff5d, because
Watcom compiler hangs. But now we can enable optimization.
Watcom: Add CFLAGS comment lines

Add -DDEBUG and -DUSE_BIOS_FOR_A20 as comments.
Watcom: Remove -D__MSC__

Watcom C is not MSC. It's not good to fake MSC.
Watcom: Add quiet option to wlink
Merge pull request #11 from k-takata/improve-watcom-support

Improve watcom support