Switch version.cpp from a rule product to a byproduct

As a rule product it will only be created once; as a byproduct the target will always be executed,
resulting in version.cpp being regenerated when additional commits are made, and yet util.a will
correctly be sequenced behind it.

This corrects the reverted PR #2657, which did not note version.cpp as a byproduct (hence a generated
file), which meant version.cpp would be disregarded as a missing source file on the initial build.

Author: smowton

src/util/CMakeLists.txt

@@ -30,8 +30,9 @@ endif()
 
 file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/version.cpp.in
   "const char *CBMC_VERSION=\"@CBMC_RELEASE@ (@GIT_INFO@)\";\n")
-add_custom_command(
-  OUTPUT version.cpp
+add_custom_target(
+  generate_version_cpp
+  BYPRODUCTS version.cpp
   COMMAND ${CMAKE_COMMAND}
     -D CBMC_SOURCE_DIR=${CBMC_SOURCE_DIR}
     -D CUR=${CMAKE_CURRENT_BINARY_DIR}
@@ -42,6 +43,8 @@ add_library(util
   ${sources}
   version.cpp)
 
+add_dependencies(util generate_version_cpp)
+
 generic_includes(util)
 
 target_link_libraries(util big-int langapi)