Split the LDFLAGS make variable into two parts: LDFLAGS is now used for

linking both executables and shared libraries, and we add on LDFLAGS_EX when
linking executables or LDFLAGS_SL when linking shared libraries.  This
provides a significantly cleaner way of dealing with link-time switches than
the former behavior.  Also, make sure that the various platform-specific
%.so: %.o rules incorporate LDFLAGS and LDFLAGS_SL; most of them missed that
before.  (I did not add these variables for the platforms that invoke $(LD)
directly, however.  It's not clear if we can do that safely, since for the
most part we assume these variables use CC command-line syntax.)

Per gripe from Aaron Swenson and subsequent investigation.

src/template/win32

@@ -1,4 +1,3 @@
-# This is required to link pg_dump because it finds pg_toupper() in
-# libpq and pgport
+# --allow-multiple-definition is required to link pg_dump because it finds
+# pg_toupper() in both libpq and pgport
 LDFLAGS="-Wl,--allow-multiple-definition"
-