Fix glibc 2.7 strtok_r() bug in a more permanent fashion.

Fix glibc 2.7 strtok_r() bug in a more permanent fashion.

The glibc 2.7 headers contain a bug that causes strtok_r() to segfault
in some circumstances.  Until now, we have been working around this
problem at each invocation, but this depends on the programmer to remember
to do so each time.

This commit instead adds a shim that adds a work-around to the string.h
header itself, so that it is much more difficult to miss the workaround.