Never mind, I found it.  

Anyway, just one more question: if I want to use VS2017, should I put "<compatibility>vc141", or "<compatibility>vc14.1"?  

This is what the file looks like right now (I changed the version from 1915 to 1916 and kept the last line from the default configuration file ("option.set keep-going : false ;"):

"
import option ;
 ​
using clang-win : : "C:\\Program Files\\LLVM\\bin\\clang-cl.exe" -fuse-ld=lld : ​
    <compatibility>vc141 ​
    <compileflags>-fmsc-version=1916 ​
    <compileflags>-fms-compatibility ​
    <compileflags>-fno-delayed-template-parsing ​
    <compileflags>-flto=thin ​
    <cxxflags>"/std:c++17" ​
    <cxxflags>"/Zc:forScope" ​
    <cxxflags>"/Zc:wchar_t" ​
    <cxxflags>"/EHsc" ​
    <cxxflags>"/Zc:inline" ​
    <cxxflags>"/Gd" ​
    <cxxflags>"/diagnostics:classic" ​
    <cxxflags>-Wno-unknown-argument​
    <cxxflags>-Wno-unknown-pragmas​
    <cxxflags>-Wno-macro-redefined​
    <cxxflags>-Wno-unused-variable​
    <cxxflags>-Wno-mismatched-tags​
    <cxxflags>-Wno-deprecated-declarations​
    <cxxflags>-D_UNICODE ​
    <cxxflags>-DUNICODE ​
    <cxxflags>-DBOOST_USE_WINDOWS_H ​
    <cxxflags>-DBOOST_NO_ANSI_APIS ​
    <cxxflags>-DBOOST_USE_WINAPI_VERSION=0x1000 ​
    <cxxflags>-DBOOST_USE_WINDOWS_H=1 ​
    <cxxflags>-DNOMINMAX ​
    <cxxflags>-DWIN32_LEAN_AND_MEAN ​
    <cxxflags>-D_CRT_SECURE_NO_WARNINGS​
;  ​
 ​
option.set keep-going : false ; ​
 ​"


From: Osman Zakir <osmanzakir90@hotmail.com>
Sent: Thursday, December 13, 2018 7:44 PM
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] project-config.jam File for building Boost with LLVM toolchain?
 
I remember from a previous email thread that someone on this mailing list had given me a better configuration jamfile.  I can't find it right now, so it'd be good if I could get that one again.  It had a "using clang-win" line after the "import" command.  And the <compatibility> tag, among some other things.  I don't remember all of the commands, so I want to see it again.