WAF: Add support for stopping processing before end of wscript{_*}

WAF: Add support for stopping processing before end of wscript{_*}

WAF scripts are written in Python and Python has no simple way
to stop program execution other than using exceptions.

This change adds WscriptCheckSkipped exception and its handling in
core WAF code. When any of wscript{_*} throws WscriptCheckSkipped
exception, WAF simply continues to process next wscript in queue
rather than breaking build.

WscriptCheckSkipped exception can be used to perform early bail out
of configuration/build target checks if certain dependency is not available
when the default checks are way more numerous than a check for this
particular dependency. This is to avoid 'if ...' indenting for large
blocks of existing code which also muddens git history for nothing.

Signed-off-by: Andreas Schneider <asn@samba.org>