aix: Fix SHLIB_EXPORTS reference in VPATH builds

The dependencies here aren't quite right independent of vpath builds or not,
but this at least makes vpath builds succeed. And it's pretty rare to change
the exports.txt file anyway... The referenced thread has a patch that will
clean that up further.

Discussion: https://postgr.es/m/20220820174213.d574qde4ptwdzoqz@awork3.anarazel.de

Author: anarazel

src/Makefile.shlib

@@ -307,7 +307,7 @@ $(shlib): $(OBJS) | $(SHLIB_PREREQS)
 ifeq (,$(SHLIB_EXPORTS))
 	$(MKLDEXPORT) $(stlib) $(shlib) >$(exports_file)
 else
-	( echo '#! $(shlib)'; $(AWK) '/^[^#]/ {printf "%s\n",$$1}' $(SHLIB_EXPORTS) ) >$(exports_file)
+	( echo '#! $(shlib)'; $(AWK) '/^[^#]/ {printf "%s\n",$$1}' ${srcdir}/$(SHLIB_EXPORTS) ) >$(exports_file)
 endif
 	$(COMPILER) -o $(shlib) $(stlib) -Wl,-bE:$(exports_file) $(LDFLAGS) $(LDFLAGS_SL) $(SHLIB_LINK)
 	rm -f $(stlib)