Currently add_cmake_flag(CMAKE_C_FLAGS -D_GNU_SOURCE) occurs in multiple platform configurations, for example: cmake/configs/*/linux/config.cmake.

Recommend this be refactored to a single location that uses the cmake function check_symbol_exists . The logic being that if the compiler supports that symbol, we want to use it, and it will de-duplicate the cmake files simplifying maintenance.

Steps:

1. Add check_symbol_exists for _GNU_SOURCE in a common location and update flags accordingly.
2. Remove all explicit duplicate entries in cmake config files.