ENABLE_LLVM_SHARED allows to dynamically link against libLLVM, but
libclang is still unconditionally linked statically.
Search for libclang-cpp.so, and if it is found and ENABLE_LLVM_SHARED
is set dynamically link against it.
Also expand the libstdc++ static linking check to include this new
condition.

Move two usdt related files to a new subdirectory and link them into the
final product. Introduce a cmake option to disable that feature (turning
it off currently fails to compile).

Move some of the cmake flag computation into a helper cmake file and
include that rather than inlining it.

Signed-off-by: Brenden Blanco <bblanco@gmail.com>