Fix compatibility with the current libmicrohttpd version

[barracuda156]

// https://github.com/macports/macports-ports/pull/8941/files
// Beginning with v0.9.71, libmicrohttpd changed the return type
// of most functions from int to enum MHD_Result
// https://git.gnunet.org/gnunet.git/tree/src/include/gnunet_mhd_compat.h
// proposes to define a constant for the return type so it works well
// with all versions of libmicrohttpd

The PR fixes compatibility with the current version of libmicrohttpd.

[barracuda156]

@Lazin Could you please take a look at this PR and other two:

#384
#385

In Macports the build succeeds now on Intel and PPC, with these fixes applied (before the port was broken across the board).