This change was erronously ommitted from the r185690, and attempt
to simply add the prototype to string.h has revealed that several
contributed programs defined local prototypes for strndup(), controlled
by autoconfed config.h. So, manually change #undef HAVE_STRNDUP to
#define HAVE_STRNDUP 1. Next import of the corresponding program would
regenerate config.h, overriding the changes in this commit.
No objections from: kan
