cmake: fix mingw-llvm shared linking

cmake: fix mingw-llvm shared linking

mingw-llvm can't handle the '--exclude-libs' linker flag, so
we'll have to skip it.

At the same time, cmake can't locate the boost shared libs as the
import libs are not generated when using mingw-llvm due to boost's
clang-linux.jam file. For now, we'll patch the cmake files, using
the dlls as import libs (which is allowed by mingw).

While at it, we'll avoid linking the static AND dynamic boost libs,
speeding the build.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>