Don't hardwire path to $OUT in when linking against shared library.

Use pattern magic to change $(OUT)/libmupdf.so into -L$(OUT) -lmupdf.

Makefile

@@ -63,6 +63,14 @@ TAGS_CMD = $(QUIET_TAGS) ctags -R --c-kinds=+p --exclude=platform/python --exclu
 WINDRES_CMD = $(QUIET_WINDRES) $(MKTGTDIR) ; $(WINDRES) $< $@
 OBJCOPY_CMD = $(QUIET_OBJCOPY) $(MKTGTDIR) ; $(LD) -r -b binary -z noexecstack -o $@ $<
 
+ifeq ($(shared),yes)
+LINK_CMD = $(QUIET_LINK) $(MKTGTDIR) ; $(CC) $(LDFLAGS) -o $@ \
+	$(filter-out %.$(SO),$^) \
+	$(sort $(patsubst %,-L%,$(dir $(filter %.$(SO),$^)))) \
+	$(patsubst lib%.$(SO),-l%,$(notdir $(filter %.$(SO),$^))) \
+	$(LIBS)
+endif
+
 # --- Rules ---
 
 $(OUT)/%.a :