[RFC] Use -- to forcibly end options in ninja generated build files

Closes #1222

Use end of options -- to preserve any trailing spaces
According to [this](http://www.cmake.org/cmake/help/v3.0/command/add_custom_command.html), ARGS is for backwards compatibility (even in CMake 2.8). If you put everything on the COMMAND line, does it make any difference? E.g.:

```
  add_custom_command(OUTPUT ${DEPS_BIN_DIR}/busted
    COMMAND ${DEPS_BIN_DIR}/luarocks build busted 2.0.rc3 CC=${DEPS_C_COMPILER} LD=${DEPS_C_COMPILER}
    DEPENDS luarocks)
```