It happened to me when doing the Haiku port, src/mpd failed to
be relinked properly when editing source files, and likely also
happens on win32, although I didn't try this change.

When building for windows, src_mpd_DEPENDENCIES is overriden.

Automake then disables the default version which contains all
the static libraries. In Makefile.in:
@HAVE_WINDOWS_FALSE@src_mpd_DEPENDENCIES = libmpd.a \

Instead we use EXTRA_src_mpd_DEPENDENCIES which is meant for this.