Add support for build2

[boris-kolpackov]

This patch make the library a build2 package. It includes support for building and running tests. CI results:

https://ci.stage.build2.org/?builds=libboost-predef

Notes:

• Using build2 version from stage to take advantage of the latest features and fixes (the release is imminent).

• Package/library naming follows Debian packages (if changing, do it in manifest and build/bootstrap.build).

• Manifest may need tweaking (url, email, etc).

• Only minimum .gitignore entries. Can add the full set to support in-source builds.

If build2 were the only build system...

• We could auto-generate the version headers with manifest becoming the only place where the version needs updating.

Let me know if there are any issues.

[boris-kolpackov]

build2 0.9.0 is out so this now works with the latest released version. Here are the CI results:

https://ci.cppget.org/?builds=libboost-predef

[hazelnusse]

@boris-kolpackov FYI, I see no CI results at the URL you provided.

[boris-kolpackov]

@hazelnusse Ah, they must have expired. Just re-submitted a new CI request, thanks for the heads-up.