check.h: only mark _ck_assert_failed as noreturn when using fork

check.h: only mark _ck_assert_failed as noreturn when using fork

When fork is used, _ck_assert_failed will call exit() when a
failure occurs. Marking it as noreturn makes sense in this case.

When fork() is not used, longjmp() is called, which is a type of
return. Marking it as noreturn allows the compiler (gcc) to
make assumptions which are not true. Using longjmp causes
the unit testing program to segfault.

For this reason, the function is only marked as noreturn when
using fork(). Any static source code analysis to catch issues
must be done on a system with fork() to avoid possible false
positives.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@871 64e312b2-a51f-0410-8e61-82d0ca0eb02a