this reverts commit 2c1f8fd5da3306fd7c8a2267467e44eb61f12dd4. without
the _Noreturn attribute, the compiler cannot use asserts to perform
reachability/range analysis. this leads to missed optimizations and
spurious warnings.
the original backtrace problem that prompted the removal of _Noreturn
was not clearly documented at the time, but it seems to happen only
when libc was built without -g, which also breaks many other
backtracing cases.

some software simply uses static_assert if the macro is defined, and
this breaks if the compiler does not recognize the _Static_assert
keyword used to define it.

add static_assert and protect the other new C11 keyword macros
with #ifndef __cplusplus so they don't conflict with C++ keywords.

for _Noreturn functions, gcc generates code that trashes the
stack frame, and so it makes it impossible to inspect the causes
of an assert error in gdb.
abort() is not affected (i have not yet investigated why).

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.
installation docs have also been updated to reflect this change.

note that POSIX does not specify these functions as _Noreturn, because
POSIX is aligned with C99, not the new C11 standard. when POSIX is
eventually updated to C11, it will almost surely give these functions
the _Noreturn attribute. for now, the actual _Noreturn keyword is not
used anyway when compiling with a c99 compiler, which is what POSIX
requires; the GCC __attribute__ is used instead if it's available,
however.
in a few places, I've added infinite for loops at the end of _Noreturn
functions to silence compiler warnings. presumably
__buildin_unreachable could achieve the same thing, but it would only
work on newer GCCs and would not be portable. the loops should have
near-zero code size cost anyway.
like the previous _Noreturn commit, this one is based on patches
contributed by philomath.