debian: use DEB_*_MAINT_APPEND for passing additional flags.

No functional changes.

debian/nginx.rules.in

diff --git a/debian/nginx.rules.in b/debian/nginx.rules.in
index bc9e1b8..1ce0394 100755
--- a/debian/nginx.rules.in
+++ b/debian/nginx.rules.in
@@ -2,12 +2,11 @@
 
 #export DH_VERBOSE=1
 export DEB_BUILD_MAINT_OPTIONS=hardening=+all,-pie
+export DEB_CFLAGS_MAINT_APPEND=-fPIC
+export DEB_LDFLAGS_MAINT_APPEND=-pie
 DPKG_EXPORT_BUILDFLAGS = 1
 include /usr/share/dpkg/buildflags.mk
 
-ADD_CFLAGS = -fPIC
-ADD_LDFLAGS = -pie
-
 PKGS = nginx nginx-dbg
 
 BUILDDIR_nginx = $(CURDIR)/debian/build-nginx
@@ -39,12 +38,12 @@
 
 config.status.nginx: config.env.nginx
 	cd $(BUILDDIR_nginx) && \
-	CFLAGS="" ./configure %%BASE_CONFIGURE_ARGS%% --with-cc-opt="$(CFLAGS) $(ADD_CFLAGS)" --with-ld-opt="$(LDFLAGS) $(ADD_LDFLAGS)"
+	CFLAGS="" ./configure %%BASE_CONFIGURE_ARGS%% --with-cc-opt="$(CFLAGS)" --with-ld-opt="$(LDFLAGS)"
 	touch $@
 
 config.status.nginx_debug: config.env.nginx_debug
 	cd $(BUILDDIR_nginx_debug) && \
-	CFLAGS="" ./configure %%BASE_CONFIGURE_ARGS%% --with-cc-opt="$(CFLAGS) $(ADD_CFLAGS)" --with-ld-opt="$(LDFLAGS) $(ADD_LDFLAGS)" --with-debug
+	CFLAGS="" ./configure %%BASE_CONFIGURE_ARGS%% --with-cc-opt="$(CFLAGS)" --with-ld-opt="$(LDFLAGS)" --with-debug
 	touch $@
 
 build-arch.%: config.status.%