dll-path also affects MSVC compiler on Windows

The current documentation of `dll-path` says:

https://github.com/boostorg/build/blob/b54f53ccbca07548df52c3a3ea63af678858569d/src/tools/features/dll-feature.jam#L10-L13

This is not correct according to what I can reproduce using `Boost.Build 4.1-git` from the current Boost develop branch and `toolset-msvc-14.2`

I have `%HOME%\user-config.jam` with `dll-path` locations specified in the `project` requirements:

```
project
: requirements
32:C:/vcpkg/installed/x86-windows/bin
64:C:/vcpkg/installed/x64-windows/bin
;

using libpng
:
: C:/vcpkg/installed/x86-windows/include C:/vcpkg/installed/x86-windows/lib
: 32
;

using libpng
:
: C:/vcpkg/installed/x64-windows/include C:/vcpkg/installed/x64-windows/lib
: 64
;

...
```

Complete version of `user-config.jam` with specification of other libraries like libjpeg, libtiff, zlib is available from Boost.GIL repo [example/b2/user-config-windows-vcpkg.jam](https://github.com/boostorg/gil/blob/837888bef238e2473272c323b81f2d9d1bf6266c/example/b2/user-config-windows-vcpkg.jam).

Next, building Boost.GIL tests that require libpng (and others):

```console
b2.exe -d2 toolset=msvc-14.2 address-model=32 libs/gil/test/extension/io//simple
```

confirms the `dll-path` locations are added to `Path` environment variable:

```console
testing.capture-output bin.v2\libs\gil\test\extension\io\all_formats_test.test\msvc-14.2\debug\asynch-exceptions-on\threading-multi\all_formats_test.run

set Path=C:\vcpkg\installed\x86-windows\bin;D:\boost.win\bin.v2\libs\filesystem\build\msvc-14.2\debug\asynch-exceptions-on\threading-multi;D:\boost.win\bin.v2\libs\system\build\msvc-14.2\debug\asynch-exceptions-on\threading-multi;D:\boost.win\bin.v2\libs\test\build\msvc-14.2\debug\asynch-exceptions-on\threading-multi;C:\vcpkg\installed\x86-windows\lib;%Path%
...
**passed** bin.v2\libs\gil\test\extension\io\all_formats_test.test\msvc-14.2\debug\asynch-exceptions-on\threading-multi\all_formats_test.test
```

Next, if I remove the whole `project` definition from the `user-config.jam`, then the `Path` is not updated and I'm getting the expected run-time errors due to the DLL being missing: