D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
For commands that always output to stdout and don't have a "-o" or
"--output" or some other similar option, this 'capture' setting allows
the build to capture the result and place it in the output file.
* Use the env variable to $PATH is searched instead of hardcoded
* Enables a local python build to take priority over system python build
as commonly used by tools like virtualenv.