Have configuration templates augment, not replace, LDFLAGS.

This preserves user-specified LDFLAGS; we already kept user-specified
CFLAGS and CPPFLAGS.  Given the shortage of complaints and the fact that
any problem caused is likely to appear at build time, no back-patch.

Dag-Erling Smørgrav and Noah Misch

src/template/cygwin

@@ -6,4 +6,4 @@ SRCH_LIB="/usr/local/lib"
 # pg_toupper() etc. in both libpq and pgport
 # 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"
+LDFLAGS="$LDFLAGS -Wl,--allow-multiple-definition -Wl,--enable-auto-import"

src/template/win32

@@ -3,4 +3,4 @@
 # --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"
+LDFLAGS="$LDFLAGS -Wl,--allow-multiple-definition -Wl,--disable-auto-import"