I think that the creation of LibMNG DLL and its import library need some fixes:
1) Into the sources, you must add MNG_EXT to all functions that you want to be exported (just putting it on the prototypes into libmng.h is not sufficient).
2) Into makefile/makefile.mingwdll, add this option to LDFLAGS:
This will allow some software (like XINE) to detect if libmng is installed at configure time.
3) Into makefile/makefile.mingwdll, Remove the "-lmng" from LDFLAGS.
4) After all previous changes, the DLL can be built with:
$(LIBMNG_DLL) : $(OBJS)
gcc -shared $^ -o $@ -Wl,--out-implib,$(LIBMNG_A) $(LDFLAGS)
I hope you will find it helpful.