Add scons options for compilation verbosity

[clemisch]

Currently the compilation with scons lib or scons install is quite noisy: it prints all compiler commands and all compiler warnings. There is no flag to control it.

For development I found that annoying, mainly because it's hard to spot an error in the wall of warnings.

This PR adds scons flags verbose: true|false = false and warnigns: all|default|none = none. verbose controls if compiler commands are printed. warnings controls compiler warnings. For the latter:

• all: inherited -W flags (from python-config) + -Wextra
• default: no explicit warning-control flags
• none: suppress all warnings (only show errors) [new default]

I think it's more common to show warnings on demand rather than by default, but that's kinda subjective and I'm happy to change the default to warnings=all and verbose=true. That would mimick current behavior.

[sbillinge]

LGTM