Watcom: Suppress WMake interactive prompt on error

Add the "-e" option to build.make invocations of wmake as part of
the silencing flags.  From "wmake /?":

 -e erase files after error (no prompt)

This prevents test timeouts on error.
Merge topic 'watcom-no-prompt' into next

abd2b36 Watcom: Suppress WMake interactive prompt on error