Remove strong dependency check for mbstring

The mbstring functions that are used here can be implemented directly in PHP, (see https://github.com/nicolas-grekas/Patchwork-UTF8/blob/master/class/Patchwork/PHP/Shim/Mbstring.php )
but checking for the extension directly excludes this kind of compatibility implementations from working.
Check mb_substr instead of mb_strlen.
ext-mbstring is a *strong* suggestion.

It allows to use fake implementations of ext/mbstring.
[auto] Merge from String

    • #0d0a408 — Remove strong dependency check for mbstring (Nicolas Grekas, 2013-08-29)
    • #ce9bac2 — Check mb_substr instead of mb_strlen. (Ivan Enderlin, 2013-08-29)
    • #d1d3d10 — ext-mbstring is a *strong* suggestion. (Ivan Enderlin, 2013-08-29)

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Merge remote branch 'library-string/master'