[BuildTools] BuildTools update and migration to GitHub

[Stefan Vigerske]

Hi,

there has been a long development to upgrade BuildTools to use current 
autotools versions, simplify future maintenance, and improve 
applicability on Windows. The current master branch seems to be quite 
stable, current Ipopt releases are based on this version of BuildTools, 
and it is expected that more projects will release with a buildsystem 
that uses BuildTools master (or maybe a stable branch to be created) in 
the future.

Compared to stable/0.8. there were a number of backward-incompatible 
changes and removal of features which maintenance effort did not seem to 
justify its usability. Large parts of coin.m4 were rewritten.

Further, BuildTools has moved to GitHub: 
https://github.com/coin-or-tools/BuildTools
The svn repository is still available and branch stable/0.8 in the svn 
repo is almost in sync with the GitHub version, but future commits will 
only go to the git repository.
The master branch only exists on GitHub.
The ticket system in Trac should be read-only and all tickets were 
migrated into GitHub issues.
The wiki in Trac is also no longer updated. However, documentation for 
BuildTools users (usually developers of COIN-OR projects) has been 
updated to be consistent with BuildTools master and is now available at 
https://coin-or-tools.github.io/BuildTools.

Best,
Stefan