Revert to using --enable-auto-import in Cygwin builds.

Disabling auto-import requires that all libraries we use be careful about
declspecs for exported variables; and it seems they aren't.  This means
that Cygwin will not give us useful info about missing PGDLLIMPORT markers;
but it's probably sufficient that MSVC and Mingw builds do.

src/template/cygwin

@@ -4,5 +4,6 @@ SRCH_LIB="/usr/local/lib"
 
 # --allow-multiple-definition is required to link pg_dump because it finds
 # pg_toupper() etc. in both libpq and pgport
-# --disable-auto-import is to ensure we get MSVC-like linking behavior
-LDFLAGS="-Wl,--allow-multiple-definition -Wl,--disable-auto-import"
+# we'd prefer to use --disable-auto-import to match MSVC linking behavior,
+# but support for it in Cygwin is too haphazard
+LDFLAGS="-Wl,--allow-multiple-definition -Wl,--enable-auto-import"