Add __attribute__((gnu_inline)) for GCC < 4.3
Add missing #define _POSIX_C_SOURCE 199309L

This also removes #include <time.h> from OFHTTPClientTests, as this
would cause <time.h> to be included without _POSIX_C_SOURCE set to
199309L and thus struct timespec would be missing.
Update copyright in a few forgotten places
Update PLATFORMS.md