boost: Fix C++11 build.

* Use default darwin toolset.
  - Used to set toolset as clang in C++11 build, which will override
    darwin settings and fail to increase template depth which is
    required for some of the libraries such as Boost.Log.
* Remove unnecessary "-fPIC" build flag.

Closes #22942.
hubflow 1.5.2

Closes #22962.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>