Removed the use_gnu_ext option as well as fallback paths for compilers
that don't support GNU extensions. To my knowledge, none of those
compilers support C11 to a sufficient extent to compile Taisei anyway,
and those fallbacks are very poorly tested.
Pedantic warnings are now disabled, and extensions that are common to
reasonably recent versions of GCC and clang are permitted to be relied
on (list of allowed extensions TBA).
