Add _FORTIFY_SOURCE to default compiler options for linux template

Many distributors use this, so we might as well see the warnings as
well.

src/template/linux

@@ -3,6 +3,9 @@
 # Force _GNU_SOURCE on; plperl is broken with Perl 5.8.0 otherwise
 CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
 
+# Many distributors use this, so we might as well see the warnings as well.
+CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
+
 # If --enable-profiling is specified, we need -DLINUX_PROFILE
 PLATFORM_PROFILE_FLAGS="-DLINUX_PROFILE"