Include <strings.h> in addition to <string.h>. According to the POSIX
specification the prototypes for strcasecmp() and strncasecmp() are
declared in <strings.h>. On some systems they are also declared in
<string.h> for compatibility reasons but we shouldn't rely on that.

Define _POSIX_C_SOURCE only when it's required to do so. Some BSD
variants doesn't declare certain function prototypes otherwise.