Add a note about how to link absl::log_flags under CMake to workaround

the lack of a feature equivalent to Bazel's alwayslink=True

PiperOrigin-RevId: 598855446
Change-Id: I0bb3bc40005908106eb7a7252572e3af153a7f5c

CMake/AbseilHelpers.cmake

@@ -80,7 +80,7 @@ endif()
 #     absl::fantastic_lib
 # )
 #
-# TODO: Implement "ALWAYSLINK"
+# TODO(b/320467376): Implement "ALWAYSLINK".
 function(absl_cc_library)
   cmake_parse_arguments(ABSL_CC_LIB
     "DISABLE_INSTALL;PUBLIC;TESTONLY"

absl/log/CMakeLists.txt

@@ -461,6 +461,11 @@ absl_cc_library(
   PUBLIC
 )
 
+# Warning: Many linkers will strip the contents of this library because its
+# symbols are only used in a global constructor. A workaround is for clients
+# to link this using $<LINK_LIBRARY:WHOLE_ARCHIVE,absl::log_flags> instead of
+# the plain absl::log_flags.
+# TODO(b/320467376): Implement the equivalent of Bazel's alwayslink=True.
 absl_cc_library(
   NAME
     log_flags