Add setup of system include directories to language related macro to remove extra lines for C and CXX.
System include directories are always same for both languages (they are defined per platform).
DOS 16-bit and 32-bit target support files for cross-compilation by
OpenWatcom.
It is used if CMAKE_SYSTEM_NAME=DOS is defined.
If CMAKE_SYSTEM_PROCESSOR=I86 is defined then 16-bit DOS target is used,
otherwise 32-bit DOS target is used.