Add OWTOOLS environment variable to setvars script to identify bootstrap compiler for OW build system, because OW build system is not able determine which toolchain is used.
OWUSENATIVETOOLS variable is removed because it is useles now.
Change appropriate make file to use this variable to switch compiler options for appropriate toolchain
supproted values are
WATCOM   - use Open Watcom for bootstrap build (define __WATCOM__ macro )
VISUALC  - use Microsoft Visual C++ for bootstrap build (define __MSVC__ macro )
GCC      - use GNU gcc compiler for bootstrap buil (define __GCC__ macro )
CLANG    - use Clang compiler for bootstrap build (define __CLANG__ macro )
INTEL    - use Intel C++ compiler for bootstrap build (define __INTEL__ macro )
if it is not defined or unsupprted value is used then default to WATCOM