Boost.Build doesn't create config.log when --build-dir is specified

[alexsharoff@…]

Platform: Windows

How to reproduce:

1. Extract Boost sources to some directory, say D:\boost
2. Bootstrap Boost as usual
3. Build Boost like this: b2 --build-dir=D:\boost-build stage

Build process will not produce configuration log in D:\boost-build\boost\bin.v2\config.log, but will spew all configuration messages and errors to console.

What I've found:

• in tools\build\src\build-system.jam, line 678: $(first-project-root).build-dir yields "/D:/boost-build/boost/bin.v2". With a slash at the beginning it's not a valid Windows path.
• in tools\build\src\build-system.jam, line 679: set-log-file rule silently fails
• in tools\build\src\build\configure.jam, line 280: FILE_OPEN is unable to create "/D:/boost-build/boost/bin.v2/config.log"

I don't have enough Boost.Build knowledge to provide a reasonable patch, but the problem seems to stem from build-dir project attribute prepending a slash to its value on Windows.

[alexsharoff@…]

A hacky patch that I made to fix the problem on my end