debian: Fix build with old versions of dpkg-buildflags.

dpkg-buildflags has not always supported --export=configure, but commit
6c2d4c8780 (debian: Apply hardening options to build.) used it
unconditionally, causing the build to fail on old Debian distributions.
This fixes the problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/debian/rules b/debian/rules
index 608f127..c11a642 100755 (executable)
--- a/debian/rules
+++ b/debian/rules
@@ -28,6 +28,12 @@ else
 CFLAGS += -O2
 endif
 
+# Old versions of dpkg-buildflags do not understand --export=configure.
+# When dpkg-buildflags does not understand an option, it prints its full
+# --help output on stdout, so we have to avoid that here.
+buildflags := $(shell if dpkg-buildflags --export=configure >/dev/null 2>&1; \
+                     then dpkg-buildflags --export=configure; fi)
+
 configure: configure-stamp
 configure-stamp:
        dh_testdir
@@ -39,8 +45,7 @@ configure-stamp:
                test -e Makefile || \
                ../configure --prefix=/usr --localstatedir=/var --enable-ssl \
                        --sysconfdir=/etc CFLAGS="$(CFLAGS)" \
-                       $(shell dpkg-buildflags --export=configure) \
-                       $(DATAPATH_CONFIGURE_OPTS))
+                       $(buildflags) $(DATAPATH_CONFIGURE_OPTS))
        touch configure-stamp
 
 #Architecture