Rebuild fish.pc as appropriate

Re-running ./configure will cause fish.pc to rebuild, in case any of the
paths changed. It looks like this actually won't rebuild the rest of
fish, but figuring out how to handle that is out of scope for this
commit.

More importantly, this will rebuild fish.pc when the version string
changes.

Makefile.in

@@ -443,8 +443,10 @@ doc.h: $(HDR_FILES)
 	cat $*.txt >>$@;
 	echo "*/" >>$@
 
-%: %.in
-	$(SED) <$@.in >$@ \
+# Depend on Makefile because I don't see a better way of rebuilding
+# if any of the paths change.
+%: %.in Makefile FISH-BUILD-VERSION-FILE
+	$(SED) <$< >$@ \
 		-e "s,@sysconfdir\@,$(sysconfdir),g" \
 		-e "s,@datadir\@,$(datadir),g" \
 		-e "s,@docdir\@,$(docdir),g" \