Add PostgreSQL version to coverage output

Also make overriding the title easier.  That helps telling where the
report came from and labeling different variants of a report.
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/Makefile.global.in

@@ -893,9 +893,12 @@ coverage: $(local_gcda_files:.gcda=.c.gcov)
 .PHONY: coverage-html
 coverage-html: coverage-html-stamp
 
+GENHTML_FLAGS = --show-details --legend
+GENHTML_TITLE = PostgreSQL $(VERSION)
+
 coverage-html-stamp: lcov_base.info lcov_test.info
 	rm -rf coverage
-	$(GENHTML) --show-details --legend --output-directory=coverage --title=PostgreSQL --num-spaces=4 --prefix=$(abs_top_srcdir) $^
+	$(GENHTML) $(GENHTML_FLAGS) -o coverage --title='$(GENHTML_TITLE)' --num-spaces=4 --prefix='$(abs_top_srcdir)' $^
 	touch $@
 
 LCOV += --gcov-tool $(GCOV)