Fix generating version.cpp

Also removes BYPRODUCT dependency on CMake 3.2

cmake/Version.cmake

@@ -37,14 +37,16 @@ if ("$Format:$" STREQUAL "")
   write_static_version_header("release")
 elseif (GIT_FOUND OR Git_FOUND)
   message(STATUS "Found Git: ${GIT_EXECUTABLE}")
-  add_custom_target(genversion ALL
+  add_custom_command(
+    OUTPUT            "${CMAKE_BINARY_DIR}/version.cpp"
     COMMAND           "${CMAKE_COMMAND}"
                       "-D" "GIT=${GIT_EXECUTABLE}"
                       "-D" "TO=${CMAKE_BINARY_DIR}/version.cpp"
                       "-P" "cmake/GenVersion.cmake"
-    BYPRODUCTS        "${CMAKE_BINARY_DIR}/version.cpp"
     WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
 else()
   message(STATUS "WARNING: Git was not found!")
   write_static_version_header("unknown")
 endif ()
+add_custom_target(genversion ALL
+  DEPENDS "${CMAKE_BINARY_DIR}/version.cpp")