Fully functional driver (smlrcc)

- Support default system header and library paths:
-- derived from the environment variable SMLRC (Linux: $SMLRC/include
and $SMLRC/lib; DOS/Windows: %SMLRC%/include and %SMLRC%/lib)
-- Linux user home directory ($HOME/smlrc/include, $HOME/smlrc/lib)
-- Linux user local directory (/usr/local/smlrc/include,
/usr/local/smlrc/lib)
-- DOS/Windows path to smlrcc (%EXE_PATH%/../include, %EXE_PATH%/../lib)
- Support overriding default system library path with the '-SL <path>'
option
- Add a hello-world test compilable for all platforms in all mode(l)s
- Add dummy (empty) system headers and libraries