[Patch 1/3] Use manual regex algorithm switching

[Tim Shen <timshen91 at gmail dot com>]

We used _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT to control which
algorithm we should use; so when compiling a regex, the number of
quantifiers(*, +, ?) will be counted and will be used for the
algorithm switching.

However, I do think give user the manual switch may make things
simpler: _GLIBCXX_REGEX_USE_THOMPSON_NFA. By default we'll use the DFS
executor, which Boost and libc++ have (only, I believe).

Users who know what they are doing and willing to pay for what they
get may switch to the Thompson NFA (we call it BFS approach) by
defining this macro.

A regex that contains back references will never be executed by a Thompson NFA.

The whole patch series are booted and tested, but this one is not
seperatly tested.

Thanks!


-- 
Regards,
Tim Shen

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