Define cmake target as PIC if BUILD_SHARED_LIBS

What language does this apply to?
Generated C/C++ code and the specific cmake target.

Describe the problem you are trying to solve.
When I link to the generated cmake target and set the BUILD_SHARED_LIBS variable globally I get linker errors because the protobuf created code is not relocateable.

Describe the solution you'd like
In my user code I use something like the following:

# ensure relocateable code
if (BUILD_SHARED_LIBS)
  set_property(TARGET ${PROTO_TARGET} PROPERTY POSITION_INDEPENDENT_CODE TRUE)
endif ()

But this could be integrated in the cmake part of this project directly.

Describe alternatives you've considered
It's not a big problem. But may be helpful for people that do not have that much cmake experience.

Additional context
Nothing.

Start in the project's CMake configuration where the generated target is defined, and inspect how BUILD_SHARED_LIBS is handled. Confirm the target's POSITION_INDEPENDENT_CODE behavior, then build a shared-library configuration and verify that generated C/C++ code links without relocation errors.