I have an old version of boost 1.33.1. I want to build it with a custom gcc binary whih is in a path other than default. I could
not find the page on the web so I paste the first part of the gcc-tools.jam form tools/build/v1/gcc-tools.jam.

# singleton variables...
set-as-singleton GCC_ROOT_DIRECTORY GCC_BIN_DIRECTORY GCC_INCLUDE_DIRECTORY GCC_STDLIB_DIRECTORY ;

flags gcc GCC_BIN_DIRECTORY : $(GCC_BIN_DIRECTORY) ;
flags gcc GCC_INCLUDE_DIRECTORY : $(GCC_INCLUDE_DIRECTORY) ;
flags gcc GCC_STDLIB_DIRECTORY : $(GCC_STDLIB_DIRECTORY) ;

GCC_BIN_DIRECTORY ?= $(GCC_ROOT_DIRECTORY)$(SLASH)bin ;
GCC_BIN_DIRECTORY ?= "" ; # Don't clobber tool names if GCC_ROOT_DIRECTORY not set
GCC_STDLIB_DIRECTORY ?= $(GCC_ROOT_DIRECTORY)$(SLASH)lib ;

# Make sure this gets set "on" the target
flags gcc GCC_BIN_DIR : $(GCC_BIN_DIRECTORY) ;

In this part I can modify the default gcc/g++ to my own using the variables which are defined in the comment section. However I don't know How to do that. For example, I write

GCC_BIN_DIRECTORY=/opt/gcc-4.1.2/installed

But that gives me this error:
rule GCC_ROOT_DIRECTORY=/opt/gcc-4.1.2/installed unknown in module

So how can I do that?

Regards,
Mahmood