Unknown compiler version while compiling Boost with MSVC 14.0 (VS 2015)

Answer:

Latest (at the time of posting this answer) Boost 1.58 does support MSVC 14.0 Preview which was the latest MS compiler at the time of Boost 1.58 release. Now, the latest version of Visual Studio is 2015 RC which isn't covered in the boost 1.58 config file.

To stop Boost 1.58 complaining about unknown compiler version edit boost/config/compiler/visualc.hpp and replace:

// last known and checked version is 19.00.22129 (VC14 Preview):
#if (_MSC_VER > 1800 && _MSC_FULL_VER > 190022310)

with:

// last known and checked version is 19.00.22816 (VC++ 2015 RC):
#if (_MSC_VER > 1800 && _MSC_FULL_VER > 190022816)

which you can find is already done in boost repo here for upcoming Boost 1.59 release.

Also if you have previously been running Boost.Build on toolset=msvc-14.0 then delete from C:\Users\<name>\AppData\Local\Temp the following cached files:

b2_msvc_14.0_vcvarsall_x86.cmd 
b2_msvc_14.0_vcvarsall_x86_amd64.cmd 
b2_msvc_14.0_vcvarsall_x86_arm.cmd

More about that here.