build: cmake: do not build storage_proxy.o by default

in 5ce07e5, the target named "storage_proxy.o" was added for
training the build of clang. but the rule for building this target
has two flaws:

* it was added a dependency of the "all" target, but we don't need
  to build `storage_proxy.cc` twice when building the tree in the
  regular build job. we only need to build it when creating the
  profile for training the build of clang.
* it misses the include directory of abseil library. that's why we
  have following build failure when building the default target:

  ```
  [2024-08-18T14:58:37.494Z] /usr/local/bin/clang++ -DDEBUG -DDEBUG_LSA_SANITIZER -DFMT_SHARED -DSANITIZE -DSCYLLA_BUILD_MODE=debug -DSCYLLA_ENABLE_ERROR_INJECTION -DSEASTAR_API_LEVEL=7 -DSEASTAR_DEBUG -DSEASTAR_DEBUG_PROMISE -DSEASTAR_DEBUG_SHARED_PTR -DSEASTAR_DEFAULT_ALLOCATOR -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SHUFFLE_TASK_QUEUE -DSEASTAR_SSTRING -DSEASTAR_TYPE_ERASE_MORE -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"Debug\" -I/jenkins/workspace/scylla-master/scylla-ci/scylla -I/jenkins/workspace/scylla-master/scylla-ci/scylla/seastar/include -I/jenkins/workspace/scylla-master/scylla-ci/scylla/build/seastar/gen/include -I/jenkins/workspace/scylla-master/scylla-ci/scylla/build/seastar/gen/src -I/jenkins/workspace/scylla-master/scylla-ci/scylla/build/gen -g -Og -g -gz -std=gnu++23 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/jenkins/workspace/scylla-master/scylla-ci/scylla=. -march=westmere -Xclang -fexperimental-assignment-tracking=disabled -U_FORTIFY_SOURCE -Werror=unused-result -fstack-clash-protection -fsanitize=address -fsanitize=undefined -fno-sanitize=vptr -MD -MT service/CMakeFiles/storage_proxy.o.dir/Debug/storage_proxy.cc.o -MF service/CMakeFiles/storage_proxy.o.dir/Debug/storage_proxy.cc.o.d -o service/CMakeFiles/storage_proxy.o.dir/Debug/storage_proxy.cc.o -c /jenkins/workspace/scylla-master/scylla-ci/scylla/service/storage_proxy.cc
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/service/storage_proxy.cc:17:
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/db/commitlog/commitlog.hh:19:
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/db/commitlog/commitlog_entry.hh:15:
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/mutation/frozen_mutation.hh:15:
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/mutation/mutation_partition_view.hh:16:
  [2024-08-18T14:58:37.495Z] In file included from /jenkins/workspace/scylla-master/scylla-ci/scylla/build/gen/idl/mutation.dist.impl.hh:14:
  [2024-08-18T14:58:37.495Z] /jenkins/workspace/scylla-master/scylla-ci/scylla/serializer_impl.hh:20:10: fatal error: 'absl/container/btree_set.h' file not found
  [2024-08-18T14:58:37.495Z]    20 | #include <absl/container/btree_set.h>
  [2024-08-18T14:58:37.495Z]       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  [2024-08-18T14:58:37.495Z] 1 error generated.
  ```
* if user only enables "dev" mode, we'd have:
  ```
  CMake Error at service/CMakeLists.txt:54 (add_library):
  No SOURCES given to target: storage_proxy.o
  ```

so, in this change, we

* exclude this target from "all"
* link this target against abseil header library, so it has access
  to the abseil library. please note, we don't need to build an
  executable in this case, so the header would suffice.
* add a proxy target to conditionally enable/disable this target.
  as CMake does not support generator expression in `add_dependencies()`
  yet at the time of writing.
  see https://gitlab.kitware.com/cmake/cmake/-/issues/19467

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

Closes scylladb#20195

service/CMakeLists.txt

@@ -51,15 +51,17 @@ target_link_libraries(service
 check_headers(check-headers service
   GLOB_RECURSE ${CMAKE_CURRENT_SOURCE_DIR}/*.hh)
 
-add_library(storage_proxy.o
-  OBJECT
-    $<$<CONFIG:RelWithDebInfo,Debug>:storage_proxy.cc>)
+add_library(storage_proxy.o OBJECT EXCLUDE_FROM_ALL
+    storage_proxy.cc)
 target_include_directories(storage_proxy.o
   PRIVATE
     ${CMAKE_SOURCE_DIR})
 target_link_libraries(storage_proxy.o
+  absl::headers
   Seastar::seastar
   xxHash::xxhash
   idl)
+add_custom_target(maybe-storage-proxy
+  DEPENDS $<$<CONFIG:RelWithDebInfo,Debug>:storage_proxy.o>)
 add_dependencies(compiler-training
-  storage_proxy.o)
+  maybe-storage-proxy)