Python path detection incorrect on macOS + brew + clang, but disallowed to pass multiple lib paths via jamfile to fix issue

Hello,

When building Boost on macOS Mojave with Homebrew and LLVM+Clang 7.0.0:

First:
- Install Homebrew
- brew install llvm (includes clang 7.0.0)
- brew install python@3
- configure clang and environment correctly

Next, build boost:
1) `./bootstrap.sh --with-toolset=clang --with-python=/usr/local/bin/python3 --with-python-version=3.7`
2) Small issue: This will cause the build to fail because you also have to provide "--with-python-root=/usr/local/bin/python3 " otherwise path detection is incorrect. Providing the path to the binary twice might be confusing for the user.

So fixing that:

1) `./bootstrap.sh --with-toolset=clang --with-python=/usr/local/bin/python3 --with-python-root=/usr/local/bin/python3 --with-python-version=3.7`

Now Python paths will be detected incorrectly. (Build will fail later on):

```
notice: [python-cfg] Configuring python...
notice: [python-cfg] user-specified version: "3.7"
notice: [python-cfg] user-specified cmd-or-prefix: "/usr/local/bin/python3"
notice: [python-cfg] Checking interpreter command "/usr/local/bin/python3"...
notice: [python-cfg] running command '/usr/local/bin/python3 -c "from sys import *; print('version=%d.%d\nplatform=%s\nprefix=%s\nexec_prefix=%s\nexecutable=%s' % (version_info[0],version_info[1],platform,prefix,exec_prefix,executable))" 2>&1'
notice: [python-cfg] ...requested configuration matched!
notice: [python-cfg] Details of this Python configuration:
notice: [python-cfg] interpreter command: "/usr/local/bin/python3"
notice: [python-cfg] include path: "/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/include/python3.7"
notice: [python-cfg] library path: "/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/config" "/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib"
```

- The detected "include path" path is incorrect, because it is missing the "m in "python3.7m"
The correct path is `/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/include/python3.7m`

- The detected config path of the "library path" is incorrect. The correct config path should be:
`/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/config-3.7m-darwin`
It is using only 'config' and not 'config-3.7m-darwin'

Therefore I modify the project-config.jam file to add the paths manually:

```
using python
: 3.7
: /usr/local/bin/python3
: /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/include/python3.7m
: /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/config-3.7m-darwin
/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib
;
```

However this produces a build error:
```
*** argument error
* rule python.init ( version ? : cmd-or-prefix ? : includes * : libraries ? : condition * : extension-suffix ? )
* called with: ( 3.7 : /usr/local/bin/python3 : /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/include/python3.7m : /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/config-3.7m-darwin
/usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib : : : : )
* extra argument /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7/lib
```

Because only a single argument is allowed to be passed via libraries.
But manually replacing all incorrectly detected paths requires passing multiple paths.