<netinet/tcp.h> <linux/tcp.h>

In general, those <linux/*> headers are not intended for direct inclusion by apps. If they're needed, one of the standard headers will generally include them, or else will just duplicate the info from them, perhaps in a slightly different way.
But, glibc and the rest of user-space is working off those standard headers, so those are the ones you want to be using.
The <linux/*> ones are typically straight out of the kernel source, and often aren't appropriate for user-space.
                    -- referenced here.

So <netinet/tcp.h> <netinet/udp.h> <netinet/icmp.h> are what we want.

If on x86 or x86_64, little-endian will set to macro __BIT_ORDER in <endian.h>.
In <netinet/tcp.h>:

 # else /* !__FAVOR_BSD */
struct tcphdr
  {
    u_int16_t source;
    u_int16_t dest;
    u_int32_t seq;
    u_int32_t ack_seq;
#  if __BYTE_ORDER == __LITTLE_ENDIAN
    u_int16_t res1:4;
    u_int16_t doff:4;
    u_int16_t fin:1;
    u_int16_t syn:1;
    u_int16_t rst:1;
    u_int16_t psh:1;
    u_int16_t ack:1;
    u_int16_t urg:1;
    u_int16_t res2:2;
#  elif __BYTE_ORDER == __BIG_ENDIAN
    u_int16_t doff:4;
    u_int16_t res1:4;
    u_int16_t res2:2;
    u_int16_t urg:1;
    u_int16_t ack:1;
    u_int16_t psh:1;
    u_int16_t rst:1;
    u_int16_t syn:1;
    u_int16_t fin:1;
#  else
#   error "Adjust your  defines"
#  endif
    u_int16_t window;
    u_int16_t check;
    u_int16_t urg_ptr;
};
# endif /* __FAVOR_BSD */

I've tested and checked that __BYTE_ORDER == __LITTLE_ENDIAN.