build: cmake: generate version files with add_custom_command()

instead of using execute_process(), let's use add_custom_command()
to generate the SCYLLA-{VERSION,RELEASE,PRODUCT}-FILE, so that we
can let other targets depend on these generated files. and generate
them on demand.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

cmake/add_version_library.cmake

@@ -4,13 +4,23 @@
 function(add_version_library name source)
   set(version_file ${CMAKE_CURRENT_BINARY_DIR}/SCYLLA-VERSION-FILE)
   set(release_file ${CMAKE_CURRENT_BINARY_DIR}/SCYLLA-RELEASE-FILE)
-  execute_process(
+  set(product_file ${CMAKE_CURRENT_BINARY_DIR}/SCYLLA-PRODUCT-FILE)
+  set(scylla_version_files
+    ${version_file}
+    ${release_file}
+    ${product_file})
+  add_custom_command(
+    OUTPUT ${scylla_version_files}
     COMMAND ${CMAKE_SOURCE_DIR}/SCYLLA-VERSION-GEN --output-dir "${CMAKE_CURRENT_BINARY_DIR}"
     WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
+  add_custom_target(scylla-version
+    DEPENDS ${scylla_version_files})
+
   file(STRINGS ${version_file} scylla_version)
   file(STRINGS ${release_file} scylla_release)
 
   add_library(${name} OBJECT ${source})
+  add_dependencies(${name} scylla-version)
   target_compile_definitions(${name}
     PRIVATE
       SCYLLA_VERSION=\"${scylla_version}\"