scons: 2.5.1 -> 3.0.0

"SCons release 3.0.0 now available from the download page at
SourceForge. This release should be used instead of 2.5.1. This release
fixes several issues.  TThis will be the first release to support Python
versions earlier than 2.7 as well as 3.5+."

"NOTE: This is a major release.  You should expect that some targets may
rebuild when upgrading.  Significant changes in some python action
signatures. Also switching between PY 2.7 and PY 3.5, 3.6 will cause
rebuilds."