debian/rules: Enable all hardening flags.

debian/rules

 #!/usr/bin/make -f
 
-export CPPFLAGS:=$(shell dpkg-buildflags --get CPPFLAGS)
-export CFLAGS:=$(shell dpkg-buildflags --get CFLAGS)
-export CXXFLAGS:=$(shell dpkg-buildflags --get CXXFLAGS)
-export LDFLAGS:=$(shell dpkg-buildflags --get LDFLAGS)
+export DEB_BUILD_MAINT_OPTIONS = hardening=+all
+DPKG_EXPORT_BUILDFLAGS = 1
+include /usr/share/dpkg/buildflags.mk
 
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