Cmake revert to using ${Protobuf_LIBRARIES}

Instead of protobuf::libprotobuf, which is currently
treated too inconsistently by CMake.
Also remove the exported find_dependency.

gtirbConfig.cmake.in

@@ -7,5 +7,3 @@ function(check_gtirb_branch GTIRB_BRANCH_NAME)
   endif()
 endfunction()
 include("${CMAKE_CURRENT_LIST_DIR}/gtirbTargets.cmake")
-include(CMakeFindDependencyMacro)
-find_dependency(Protobuf @Protobuf_VERSION@)

src/CMakeLists.txt

@@ -127,11 +127,15 @@ target_include_directories(
   ${PROJECT_NAME} PRIVATE $<BUILD_INTERFACE:${CMAKE_BINARY_DIR}/src/>
 )
 
+# NOTE (2020-12): tried using protobuf::libprotobuf to get relocatable exports,
+# but it is apparently treated inconsistently between static and shared builds.
+# For simplicity, we revert to using ${Protobuf_LIBRARIES} for now.
+
 target_link_libraries(
   ${PROJECT_NAME}
   PUBLIC ${SYSLIBS}
          ${Boost_LIBRARIES}
-         protobuf::libprotobuf
+         ${Protobuf_LIBRARIES}
          # Link in this static lib, but don't make it a transitive dependency of
          # TestGTIRB, etc
   PRIVATE gtirb_proto

src/gtirb/proto/CMakeLists.txt

@@ -45,7 +45,7 @@ endforeach(PROTO_H)
 # Add proto library target
 add_library(gtirb_proto STATIC ${PROTO_CPP_SOURCES} ${PROTO_COPIED_HEADERS})
 
-target_link_libraries(gtirb_proto ${Boost_LIBRARIES} protobuf::libprotobuf)
+target_link_libraries(gtirb_proto ${Boost_LIBRARIES} ${Protobuf_LIBRARIES})
 
 target_compile_definitions(gtirb_proto PRIVATE GTIRB_${PROJECT_NAME}_EXPORTS)