Passing defines to b2 ?

Hi.

I'm currently exploring options for how I build all my packages
(linux, basically Beyond Linux From Scratch). After successfully using

./b2 stage cxxflags="$CXXFLAGS" threading=multi link=shared -d+2

with CXXFLAGS of initially -O2 -march=native, later reduced to
 -march=native to keep the default -O3, I started to look at hardening
options to find out where the problems are, and if any cost is
worthwhile. And no,
-march=native is not necessarily a good idea, but in verbose builds it
tells me if my flags are being used.

Now I'm looking at the first of the cheap hardening options (on gcc) :
-D_FORTIFY_SOURCE=2 but b2 has a strong dislike of  -D.

After looking around, it appears that this needs to be in a jam file,
possibly in a form like <define>\_FORTIFY\_SOURCE\=2 but I'm making no
progress on this. Also, I could not find any guide to whether the
underscores need to be escaped, I just started with those and found
that the = definitely needed to be escaped. I've created
user-config.jam:

using gcc : : : <compileflags>-march=native<compileflags>_FORTIFY_SOURCE\=2 ;
project user-config ;

(using gcc through to 2; is all on one line, gmail insists on breaking it up)

Any guidance, please ?
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