Refactor DLSUFFIX handling

Move DLSUFFIX from makefiles into header files for all platforms.
Move the DLSUFFIX assignment from src/makefiles/ to src/templates/,
have configure read it, and then substitute it into Makefile.global
and pg_config.h.  This avoids the need for all makefile rules that
need it to locally set CPPFLAGS.  It also resolves an inconsistent
setup between the two Windows build systems.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/2f9861fb-8969-9005-7518-b8e60f2bead9@enterprisedb.com

src/template/cygwin

@@ -13,3 +13,5 @@ CFLAGS_SL=""
 # we'd prefer to use --disable-auto-import to match MSVC linking behavior,
 # but support for it in Cygwin is too haphazard
 LDFLAGS="$LDFLAGS -Wl,--allow-multiple-definition -Wl,--enable-auto-import"
+
+DLSUFFIX=".dll"

src/template/hpux

@@ -25,3 +25,10 @@ case $host in
 	fi
     ;;
 esac
+
+case $host_cpu in
+  ia64)
+    DLSUFFIX=".so";;
+  *)
+    DLSUFFIX=".sl";;
+esac

src/template/win32

@@ -7,3 +7,5 @@ CFLAGS_SL=""
 # pg_toupper() etc. in both libpq and pgport
 # --disable-auto-import is to ensure we get MSVC-like linking behavior
 LDFLAGS="$LDFLAGS -Wl,--allow-multiple-definition -Wl,--disable-auto-import"
+
+DLSUFFIX=".dll"