Rcently in net-libs/libmicrohttpd package from 0.9.23 and up the options have been enabled "--enable-bauth --enable-dauth" that lead to xbmc fail to compile with webserver flag or stop working properly when that flag for xbmc have been presiously turned on and libmicrohttpd package have been updated in the mean time.
XBMC upstream is yet to look in to those new features that seem to change the behaviour as even git version does not iclude work for it so for the time being.
I propose those features disabled for now or hide them behind use flag so others not hit with similar compile error still may benefit from those new options
very welcome addion would also be to the xbmc ebuild checking for appropriate use flag being disabled within libmicrohttpd
more on the issue:
*** This bug has been marked as a duplicate of bug 449082 ***