[boost] [tools] How to use 1.58.0/bjam --toolset=sun for specified stdlib feature?

Subject: [boost] [tools] How to use 1.58.0/bjam --toolset=sun for specified stdlib feature?
From: Sergey Sprogis (sergey.sprogis_at_[hidden])
Date: 2015-04-18 12:19:29

Running boost regression unit tests I found it convenient to direct 
output from every single tested library into a separate logfile. To do 
that, my testing script (bash) at first extracts the list of all 
<library>/test lines from status/Jamfile.v2, and puts them into another 
test_suites.txt file. Then it runs all extracted test suites like this:

my_res_dir= `pwd`
for dir in `cat test_suites.txt`; do
   dir1=`echo $dir | sed "s/\//\./g"`
   cd  $BOOST_ROOT/libs/$dir
   bjam --toolset=sun variant=release > $my_res_dir/$dir1.log 2>&1
done

All that worked fine until boost-1.58 in which 
tools/build/src/tools/sun.jam file was
modified so that instead of single standard library it now allows the 
possibility of
using any of 5 available libraries.

The question is, how exactly should I define bjam's options in my bash 
script shown above, if for example, I want to use gnu library in c++03 mode?
The proper  code inside sun.jam file looks like this:

feature.extend stdlib : gnu ;
feature.compose <stdlib>gnu
     : <cxxflags>-std=c++03 <linkflags>-std=c++03
     ;

but how should I tell to bjam to use this specific library?

Thanks,

Sergey