Added -oh compilation switch (#88)

Added -oh compilation switch #88 (https://github.com/open-watcom/open-watcom-v2/issues/88)
Acording to the documentation, the -oh flag (enable repeated optimizations -longer compiles-), will generate a better code, at the cost of extra compile times.
Most of the time, OpenWatcom is using /oax or /oaxt, and maybe adding /oh will be worth, at least in some parts like the clib.
Did not counted the time spent for a full build, but it is not noticeable longer than without -oh
I have executed regression tests run properly.