Work around FindGMock/Catkin target name clash (#1011)

Handle case with multiple invocations (avoid pasting ${CMAKE_THREAD_LIBS_INIT} multiple times).
Also check if target 'gmock' already exists before building GMock to avoid failures if GMock was already
find_packaged() from another subproject.

FIXES=https://github.com/googlecartographer/cartographer_ros/issues/776

cmake/modules/FindGMock.cmake

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+if(NOT GMock_FOUND)
   find_path(GMOCK_INCLUDE_DIRS gmock/gmock.h
     HINTS
       ENV GMOCK_DIR
@@ -52,7 +53,9 @@ if(NOT GMOCK_LIBRARIES)
     )
     if(GMOCK_SRC_DIR)
       # If src version found, build it.
+      if(NOT TARGET gmock)
         add_subdirectory(${GMOCK_SRC_DIR} "${CMAKE_CURRENT_BINARY_DIR}/gmock")
+      endif()
       # The next line is needed for Ubuntu Trusty.
       set(GMOCK_INCLUDE_DIRS "${GMOCK_SRC_DIR}/gtest/include")
       set(GMOCK_LIBRARIES gmock_main)
@@ -62,8 +65,11 @@ endif()
   # System-wide installed gmock library might require pthreads.
   find_package(Threads REQUIRED)
   list(APPEND GMOCK_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+endif()
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(GMock DEFAULT_MSG GMOCK_LIBRARIES
                                   GMOCK_INCLUDE_DIRS)
 
+# Mitigate build issue with Catkin
+set(GMOCK_FOUND FALSE)