[Patch] libstdc++/16612

[Paolo Carlini]

Hi all,

the below is what I have tested (with Benjamin's help) as a stop-gap fix
for the applications outlined in PR 16612: indeed, we have tested it
together with the soon-to-be-contributed shared memory allocator.

If --enable-fully-dynamic-string is passed at build time, the empty-string
optimization is completely disabled: this means that there are no major
performance disadvantages after construction time, only the empty string
construction becomes slower, roughly the same speed of a generic size-1 or
2 construction, in other terms. This seems sufficient for the applications
cited in the PR.

For v7, where the ABI can be breaked, *many* other options are possible,
which we'll be exploring in the next weeks...

I will wait 'til tomorrow in case of problematic comments.

Paolo.

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