llvm: build as a static library by default

This change inverts the default settings for building LLVM as a shared
library.  According to LLVM CMake building guide, the default setting is
to not build shared libraries and is not recommended
(http://llvm.org/docs/CMake.html).  Cross-verifying LLVM's 'configure'
script indicates that the default is to not build shared libraries as
well.

Beyond keeping the Formula 1-to-1 with the default LLVM build settings,
this change removes the burden to have one's @rpath be set appropriately
when building with Homebrew's llvm+clang as a shared library.

Commit 0b50110107ea2998e65011ec31ce45931b446dab introduced enforcing
that LLVM be built as shared library for the Python bindings.  However,
when running with '--with-python', it was observed that this flag was
not passed to LLVM's '/.configure' - this appears a bug and probably
requires further investigation.