Watcom: Fix creation of DLLs without any exported symbols

Create each DLL import library by passing "option implib=..." to the
linker for its SHARED library.  This works even when there are no
symbols to be exported.  Leave the option out for MODULE libraries
because we do not need an import library for them.  For executables,
retain the separate invocation of wlib because we want an import
library only when the ENABLE_EXPORTS property is set, and in that
case the project should provide symbols.

Suggested-by: J Decker <d3ck0r@gmail.com>
Merge topic 'watcom-dll-no-symbols' into next

00503d38 Watcom: Fix creation of DLLs without any exported symbols