Make DLSUFFIX easily discoverable by build scripts

This will enable things like the buildfarm client to discover more
reliably if certain libraries have been installed.

Discussion: https://postgr.es/m/859e7c91-7ef4-d4b4-2ca2-8046e0cbee09@2ndQuadrant.com

Backpatch to all live branches.

src/Makefile.global.in

@@ -472,6 +472,11 @@ ifeq ($(enable_rpath), yes)
 LDFLAGS += $(rpath)
 endif
 
+# Show the DLSUFFIX to build scripts (e.g. buildfarm)
+.PHONY: show_dl_suffix
+show_dl_suffix
+	@echo $(DLSUFFIX)
+
 
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