VS: Generate ToolsVersion matching each VS version

The MSBuild version for each Visual Studio generator isn't 4.0.  With
Visual Studo 2013 the ToolsVersion moved from being tied to the .NET
framework and now has its own version number.
Merge topic 'vs2013_toolset_14369' into next

3064f80 VS: Generate ToolsVersion matching each VS version
Merge topic 'fix-OLD-CMP0021'

eef6df5 Fix OLD behavior of CMP0021.
Merge topic 'rpath-on-mac'

78cd884 OSX: Allow an empty INSTALL_NAME_DIR to override MACOSX_RPATH.
Merge topic 'doc-CPack-typo'

807a564 CPack: Fix a typo in documentation
Merge topic 'vs2013_toolset_14369'

3064f80 VS: Generate ToolsVersion matching each VS version
Merge branch 'master' into next
Merge topic 'doc-check-flags'

293ecfe CheckC*CompilerFlag: add documentation what to expect from a positive result
Merge branch 'master' into next