Don't redefine str(n)casecmp on windows unless they're missing

When we do redefine them, use inline functions instead of #define.

This fixes a latent code problem in our redefinition of these
functions, which was exposed by our refactoring: Previously, we
would #define strcasecmp after string.h was included, so nothing bad
would happen.  But when we refactored, we would sometimes #define it
first, which was a problem on mingw, whose headers contain
(approximately):

inline int strcasecmp (const char *a, const char *b)
   { return _stricmp(a,b); }

Our define turned this into:
  inline int _stricmp(const char *a, const char *b)
    { return _stricmp(a,b); }

And GCC would correctly infer that this function would loop forever,
rather than actually comparing anything.  This caused bug 26594.

Fixes bug 26594; bug not in any released version of Tor.