Drop legacy flag handling from langcore.mk and drop unused variable DEB_OPT_FLAG: dpkg-buildflags is used instead when available (since cdbs 0.4.90), which is now the case even in oldstable.
Fix cdbs_set_nondefaultvars in buildvars.mk to skip undefined variables. Drop unused variable cdbs_set_nondefaultvar.

Dropped variable verified against <http://codesearch.debian.net>.
Tidy langcore.mk: Use cdbs_set_nondefaultvars to set buildflags in dpkg-buildflags call.
Speedup buildflags resolving: call dpkg-buildflags only once.
Fix buildflags resolving in langcore.mk to only pass documented supported environemnt variables to dpkg-buildflags (other DEB_* variables may be unsafe for simple shell-quoting.
Prepare for release: Update changelog.
Tidy snippets: Reorder variables to e.g. list legacy before modern, and cdbs_curpkg-resolved later.
Tidy snippets: wrap+strip wide lines, and strip trailing whitespace.
Introduce new expansion variables cdbs_uc cdbs_lc.
Modernize python-distutils.mk installdir handling (since supported even in oldstable now): Use --install-layout=deb (and drop --prefix=/usr) by default, and use --install-lib option (not --install-purelib/--install-platlib, and only when DEB_PYTHON_DISTUTILS_INSTALLDIR_SKEL is set).  Deprecate cdbs_expand_python_distutils_installdir, and have it provide .../dist-packages dir (not .../site-packages) by default to try cope with packages relying on it for non-skel use. Closes: bug#565973. Thanks to Matthias Klose and Piotr Ożarowski.
Fix error message to talk about multiple python implementations (not packaging systems nor too specific ones).
Tidy excess whitespace.
Tidy python-vars.mk: Add/improve comments.
Tighten cdbs_make_curflavor resolving, to simplify its use.
Fix allow early override of python autotools variable. Introduce new variables DEB_CONFIGURE_SCRIPT_ENV_PYTHON cdbs_make_curpythonruntime cdbs_autotools_configure_env.
Rename cdbs_python_pysupport → cdbs_pythonsystem_pysupport. Drop the former without deprecation (unused outside cdbs).

Dropped variable verified against <http://codesearch.debian.net>.
Rename cdbs_python_pycentral → cdbs_pythonsystem_pycentral. Drop the former without deprecation (unused outside cdbs).

Dropped variable verified against <http://codesearch.debian.net>.
Rename cdbs_python2 → cdbs_pythonsystem_python2. Drop the former without deprecation (unused outside cdbs).

Dropped variable verified against <http://codesearch.debian.net>).
Rename cdbs_python3 → cdbs_pythonsystem_python3. Drop the former without deprecation (unused outside cdbs).

Dropped variable verified against <http://codesearch.debian.net>.
Refactor python snippets: Expand implementations from lists.