fixes for GCC 6:

- -Wstrict-prototypes is not available for C++, so don't try to
ignore it for C++.
- remove many _DIAGASSERT() checks against not NULL for functions
with arguments with nonnull

fixes for GCC 6:

- -Wstrict-prototypes is not available for C++, so don't try to
ignore it for C++.
- remove many _DIAGASSERT() checks against not NULL for functions
with arguments with nonnull attributes. in two cases, leave
code behind that should set defaults to "(null)".
- use -Wno-error=frame-address for i386 mcount, as it seems valid
to assume the caller will have a frame.fair

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elide strict-prototypes for unrepresentable function

Revert

Fix ANSI-C prototype error

Add __returns_twice for the attribute of the same name. This makes the
attribute explicit based on the functions GCC gives it too.

Revert previous since it was just wrong.

void f(void (*g)()); -> void f(void (*g)(void));

Remove clause 3 and 4 from TNF licenses

Sprinkle some restrict.

Merge the nathanw_sa branch.