After digging into build.bat located at BOOST_ROOT\tools\build\v2\engine, when supplying "mingw" as the toolset, the script by-passed the "guessing toolset" step and failed to define the variable "BOOST_JAM_TOOLSET_ROOT", leaving calls to gcc-related executables failed.
Now I changed the batch as follow
PUSHD "%PATH_BJAM%"
SET "PATH=%PATH_MINGW%\bin"
REM add the line below
SET "BOOST_JAM_TOOLSET_ROOT=%PATH_MINGW%\"
build.bat mingw --show-locate-target
SET PATH=%OPATH%
I manually set the BOOST_JAM_TOOLSET_ROOT variable from external batch file and this worked fine. Beware of the ending "\" character, as the build.bat inside append the path variable as this:
set "PATH=%BOOST_JAM_TOOLSET_ROOT%bin;%PATH%"
Thanks very much for any comments and suggestions.