[CMake] Make the pybind11_*_proto_caster libraries STATIC

As pybind11 does not guarantee stable ABI for its types (e.g.
pybind11::handle), these types have "hidden" visibility at least on
Linux.

This visibility is propagated to any method which has any of the pybind11
types in its parameters, which is the case for many of the public methods
in proto_cast_util.{h,cc}. Trying to link to e.g. a SHARED
pybind11_native_proto_caster library will create linker errors to to
undefined symbols for any non-trivial case.

The other approach would be to move everything which uses a pybind11
type to the headers, and only leave pybind11 agnostic methods in the
shared library. While this is trivially possible for e.g.
PyBytesAsStringView, for the majority of the methods (e.g. anything
depending on the GlobalState object) a significant refactoring would
be required.

See pybind#160.

CMakeLists.txt

@@ -61,7 +61,7 @@ target_include_directories(
 # ============================================================================
 # pybind11_native_proto_caster shared library
 add_library(
-  pybind11_native_proto_caster SHARED
+  pybind11_native_proto_caster STATIC
   # bazel: pybind_library: native_proto_caster
   pybind11_protobuf/native_proto_caster.h
   # bazel: pybind_library: enum_type_caster
@@ -103,7 +103,7 @@ target_include_directories(
 # ============================================================================
 # pybind11_wrapped_proto_caster shared library
 add_library(
-  pybind11_wrapped_proto_caster SHARED
+  pybind11_wrapped_proto_caster STATIC
   # bazel: pybind_library: wrapped_proto_caster
   pybind11_protobuf/wrapped_proto_caster.h
   # bazel: pybind_library: proto_cast_util