Update to 0.7.0.

Release notes: http://herbstluftwm.org/news.html
Notable porting changes:

• herbstluftwm has started using C++. In fact, it uses C++11, which makes us require a compiler that understands C++11.
• Set COLOR=0 when building. It disables coloured output during the build, but works around a lot of tput warnings on FreeBSD 9.x like this: tput: no terminal type specified and no TERM environmental variable.

PR: 206956
Submitted by: Zsolt Udvari <udvzsolt@gmail.com> (maintainer)