b2 configuration checks take long time

Hi,

I have a very simple libs/gil/test/image_view/Jamfile

```
import testing ;

project
    : requirements
    <include>$(BOOST_ROOT)
    <include>..
    ;

test-suite "boost-gil-test-image-view"
    :
    [ run collection.cpp ]
    ;
```

The collection.cpp is just

```
int main() {}
```

In libs/gil/test, I run `b2 collection` first time.
I run `b2 collection` again.

What may be the reason it takes 15-18 seconds until I see this?

```
Performing configuration checks
    - default address-model    : 64-bit (cached)
    - default architecture     : x86 (cached)
    - symlinks supported       : yes (cached)
```

How can I speed up the b2 startup?

I'm observing this on WSL (Ubuntu Bionic, GCC 7.3), on very decent workstation.
I have no access to Linux native host to compare if perhaps the issue is
due to WSL filesystem poor performance.

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