Fix _POSIX_C_SOURCE definition

Current code makes use of `snprintf` function defined in C99 standard
but _POSIX_C_SOURCE is set to "1" which is not correct. To access C99
definitions, this macro should be set to at least "200112L".

Also, `strncasecmp` function is being used but corresponding <strings.h>
header is not being included in miniwget.c.

Changes have been tested on FreeBSD 10.1 amd64 with default compiler
(Clang).