[CMake] Fix bug in `add_weak_symbols()` function.

[delcypher]

Summary

[CMake] Fix bug in add_weak_symbols() function.

Previously the the weak_symbols.txt files could be modified and the
build system wouldn't update the link flags automatically. Instead the
developer had to know to reconfigure CMake manually.

This is now fixed by telling CMake that the file being used to
read weak symbols from is a configure-time dependency.

[george.karpenkov]

Makes sense, I guess it has to be done at configuration stage, as flags are added during the configuration.

[delcypher]

@george.karpenkov Thanks for the review.

compiler-rt/trunk/cmake/Modules/SanitizerUtils.cmake

if(ARG_PARENT_TARGET)
       add_dependencies(${ARG_PARENT_TARGET} ${target_name}-symbols)
     endif()
   endforeach()
 endmacro()
 
 # This function is only used on Darwin, where undefined symbols must be specified
 # in the linker invocation.
 function(add_weak_symbols libname link_flags)
-  file(STRINGS "${COMPILER_RT_SOURCE_DIR}/lib/${libname}/weak_symbols.txt" WEAK_SYMBOLS)
+  set(weak_symbols_file "${COMPILER_RT_SOURCE_DIR}/lib/${libname}/weak_symbols.txt")
+  file(STRINGS  "${weak_symbols_file}" WEAK_SYMBOLS)
+  # Add this file as a configure-time dependency so that changes to this
+  # file trigger a re-configure. This is necessary so that `${link_flags}`
+  # is changed when appropriate.
+  set_property(
+    DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+    APPEND
+    PROPERTY CMAKE_CONFIGURE_DEPENDS "${weak_symbols_file}")
   set(local_link_flags ${${link_flags}})
   foreach(SYMBOL ${WEAK_SYMBOLS})
     set(local_link_flags ${local_link_flags} -Wl,-U,${SYMBOL})
   endforeach()
   set(${link_flags} ${local_link_flags} PARENT_SCOPE)
 endfunction()
 
 macro(add_sanitizer_rt_version_list name)