forcing tail/sibling call optimization

[Robert Dewar]

Note that if you want to make a warning message a formal requirement,
then you must provide a formal definition of what a warning message is,
and I do not think you want to get into that sort of thing :-)

I think there is real merit in providing fairly formal specifications
for GNU C enhancements for three reasons.

1. The attempt to produce a FFS can often reveal difficulties that are
not apparent in a less formal approach.

2. If other C compilers want to copy the feature, which I assume we
welcome, then there is a clear guide.

3. If the ISO committee wants to copy the feature, which I assume
we welcome, then there is a clear guide.