Fix #64: failed build against newer libmicrohttpd versions

[hadret]

This MR fixes broken builds against newer libmicrohttpd versions while remaining compatible with the older ones. The change was introduced back in April last year by the upstream:

Wed 08 Apr 2020 10:53:01 PM CEST
Introduce enum MHD_Result for #MHD_YES/#MHD_NO to avoid using 'int' so much.
Note that this change WILL cause compiler warnings until (most) MHD callbacks
in application code change their return type from 'int' to 'enum MHD_Result'.
That said, avoiding possible confusions of different enums is going to make
the code more robust in the future. For conditional compilation, test
for "MHD_VERSION >= 0x00097002". -CG

And here's the relevant commit: https://git.gnunet.org/libmicrohttpd.git/commit/?id=6347f514aa2388e774d5bf356df8046864e5f73c

Many thanks to @janv37 for hunting this one down 🙌🏻

[kzangeli]

Looks good to me.
Will this simple fix ever be merged?

[wferi]

Ugh, I didn't notice this PR and submitted #79 myself, which also handles the same problem.
Unfortunately is doesn't seem likely that either would be merged anytime soon. :(