It should probably use pkg-config to discover the headers rather than doing so manually.
Created attachment 178481 [details] Build log
If you have a patch or can point me at another mpd util using pkg-config I'd be glad to get this fixed, but I don't have any experience with pkg-config myself.
Created attachment 182251 [details, diff] patch against configure.ac/Makefile.am
Created attachment 182253 [details, diff] patch against g15mpd-1.0.0.ebuild there you go
in portage..