I am submitting two new patches. We can undefine the macro at two locations
1). As be-secure-openssl.c [1] was the actual file where the conflict happened so I undefined the macro here before the ssl includes. I changed the comment a little to make it understandable. I am also attaching the error generated with ninja build.
OR
2). Right after the gssapi includes in libpq-be.h
Thank you for working on this. I can confirm the undef version compiles and passes tests with 16.3.