Add some hardening to the debian build

debian/rules

@@ -7,10 +7,10 @@ export VERBOSE=1
 
 FS_CC?=gcc
 FS_CXX?=g++
-FS_CPPFLAGS?=
-FS_CFLAGS?=-g3 -O2 -fPIC
+FS_CPPFLAGS?=-D_FORTIFY_SOURCE=2
+FS_CFLAGS?=-g3 -O2 -fPIC -fstack-protector --param=ssp-buffer-size=4
 FS_CXXFLAGS?=$(FS_CFLAGS)
-FS_LDFLAGS?=
+FS_LDFLAGS?=-Wl,-z,relro
 export PATH?=/usr/lib/ccache:/usr/sbin:/usr/bin:/sbin:/bin
 export CC:=$(FS_CC)
 export CXX:=$(FS_CXX)