Make warn_unused_result applied to all function with GCC

No, you can only tell gcc to ignore all warn_unused_result flags with -Wno-unused-result but the respective -Wunused-result only sets the default (to warn only on flags). Compiling with -Wall -Wextra -pedantic should have triggered a warning if it can be activated but it doesn't, so it cannot.

Besides that, I wonder why you would want this, it is not that uncommon to ignore the result of functions, and all libraries are likely to produce tons of warnings.