On line 34 of Core.mk $(MY_ROOT)/fitz/dev_draw.c \ refers to a C file that is absent from the third party zip in the current distribution. Googling the C file found it in a different fitz tree for another code project but inserting that C file into the build tree caused lots of errors. It's not clear where this file is, or whether it should exist; doing a normal Unix-style build on the same source tree doesn't seem to cause problems with this file missing. This and a couple of other bugs look as though the Android build system for mupdf is based on a different version of mupdf than the one it's shipped with.
Fixed now. Thanks.