reference counting in string's allocator, where else?

[Benjamin Kosnik]

> We have been running into problems (core dumps) with the allocators in string not being thread safe when using the 2.95.2 and 2.95.3 gcc builds.

Keep in mind that gcc-2.9.x and gcc-3.x come with completely different 
sources for libstdc++. This list is for libstdc++-v3, or the gcc-3.x version.

-benjamin