GNAT, SJLJ and zero-cost exception handling

[Ludovic Brenta]

Hi, after reading [1] and [2] I'm a bit confused.  I'd like to know how
I should go about building GCC such that I can choose between SJLJ and
ZCX at compile time, using --rts=sjlj.  From [2] it looks like this is a
configure-time option only.  Is it necessary to configure gcc twice and
produce two different libgnats?  Is it necessary to install two
compilers?

[1]
http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gnat_ugn_unw/Exception-Handling-Control.html
[2] http://gcc.gnu.org/install/configure.html#TOC5

Thanks for any help.

-- 
Ludovic Brenta.