Endianess macros not correctly defined [closed]

I'm trying to compile grpc, and one of the targets uses /usr/include/netinet/tcp.h which contains this :

# if __BYTE_ORDER == __LITTLE_ENDIAN
        uint8_t th_x2:4;        /* (unused) */
        uint8_t th_off:4;       /* data offset */
# endif
# if __BYTE_ORDER == __BIG_ENDIAN
        uint8_t th_off:4;       /* data offset */
        uint8_t th_x2:4;        /* (unused) */
# endif

Somehow, the two conditions are fulfilled which makes the compilation fail (error: duplicate member 'th_off'). I wrote a test C program and it seems that the three macros are not correctly defined.

I tried to reinstall gcc and g++, and also downgrade them to a previous version. I also tried to reinstall the Linux headers. I finally tried to compile with clang, but it doesn't work either.

My two test computers are running Ubuntu 19.10 and the result is the same on both of them. Is there a compiler flag or a system configuration needed to define these macros?

EDIT : tcp.h is a system header. I did not write it and it's an almost fresh install of Ubuntu with a standard installation of build-essentials.

EDIT 2 : I used a test program and it works correctly in this case. It shows little:

#include <iostream>
#include <netinet/tcp.h>

int main() {
#if __BYTE_ORDER == __LITTLE_ENDIAN
    std::cout << "little\n";
#endif
#if __BYTE_ORDER == __BIG_ENDIAN
    std::cout << "big\n";
#endif
}