E.g. `-128` is packed as `d1ff80` instead of `d080`.

Requested by customer.

Building Documentation topic for issue #804

Guidelines to buidling documentation and submitting proposed changes/updates

Add some OSX-specific files to .gitignore

_Noreturn kw must appear before the function name as per C11 grammar and
clang7 is the first one to enforce that.

Ecb_noreturn macro is defined using _Noreturn keyword if C11 support is
detected; otherwise __noreturn__ gcc attribute is used. The later must
appear after the function parameters list.  These two methods to
annotate a function are inherently incompatible in terms of the token
location relative to the function declaration.

Use gcc attribute exclusively since it is always available.