Add noreturn attribute to select functions and added -Wmissing-noreturn

Add noreturn attribute to select functions and added -Wmissing-noreturn

Added the gcc attribute noreturn to a few functions that could use it
(but only if the compiler is gcc 2.5 >=),  and added the warning to
point out when a function should use noreturn.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@791 64e312b2-a51f-0410-8e61-82d0ca0eb02a