[build] ERROR: rule "[[bbv2.builtin.features.warnings]]`warnings`::" unknown in module "warnings-feature"

Hi,

(CC'ed to boost and boost-build)

Two of Boost.Geometry users reported that peculiar error, [1] and [2]

```
/home/digu_j/boost/tools/build/src/tools/features/warnings-feature.jam:27:
in load
ERROR: rule "[[bbv2.builtin.features.warnings]]`warnings`::" unknown
in module "warnings-feature".
```

and I'm having a hard time understanding what is going wrong here
and what causes this issue, on Linux.

The line 27 is a typical Jam line:

```
feature.feature warnings
: on all extra pedantic off
: incidental propagated ;
```

but this refers to something that puzzles me most

```
ERROR: rule "[[bbv2.builtin.features.warnings]]`warnings`::" unknown
```

a bit of comment block?

```
#| tag::doc[]

[[bbv2.builtin.features.warnings]]`warnings`::
```

Is this a parsing issue?
Is this related to potentially existing system-wide installation of Boost.Build?
If yes, why none of the dumps in [1] and [2]  refer to any files in  /usr?

[1] ./b2 headers attempt
https://github.com/boostorg/geometry/pull/667#issuecomment-584794370

[2] b2 attempt to build a program in libs/geometry
https://github.com/boostorg/geometry/pull/665#issuecomment-595595900

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net

_______________________________________________
Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost