Add configure check to see whether <string.h> and <strings.h> may both be

Add configure check to see whether <string.h> and <strings.h> may both be
included, and then include <strings.h> if so.  Several systems already
needed <strings.h> anyway.  Some new systems that claim to conform to the
Unix 9x "standard" do not declare str[n]casemp() in string.h, and C99
compilers will not like that.