Reinstate genhtml --prefix option for non-vpath builds

In c3d9a660, the genhtml --prefix option
was removed to get slightly better behavior for vpath builds.  genhtml
would then automatically pick a suitable prefix.  However, for non-vpath
builds, this makes the coverage output dependent on the length of the
path where the source code happens to be, leading to confusingly
arbitrary results.  So put the --prefix option back for non-vpath
builds.

src/Makefile.global.in

@@ -898,7 +898,7 @@ GENHTML_TITLE = PostgreSQL $(VERSION)
 
 coverage-html-stamp: lcov_base.info lcov_test.info
 	rm -rf coverage
-	$(GENHTML) $(GENHTML_FLAGS) -o coverage --title='$(GENHTML_TITLE)' --num-spaces=4 $^
+	$(GENHTML) $(GENHTML_FLAGS) -o coverage --title='$(GENHTML_TITLE)' --num-spaces=4 $(if $(filter no,$(vpath_build)),--prefix='$(abs_top_srcdir)') $^
 	touch $@
 
 LCOV += --gcov-tool $(GCOV)