using STANDARD_BINDIR_PREFIX and overrdes

I don't understand how -B works.  It does, and now I need to propagate
the information to cobol1.

If we invoke

    gcobol -B/tmp

it will invoke /tmp/cobol1, magically.  We don't do anything to make
that happen in gcc/gcobolspec.cc.

Without -B, it seems we expect to find STANDARD_BINDIR_PREFIX/cobol1.

Is the value of STANDARD_BINDIR_PREFIX assigned to a variable, and then
modified by -B and/or GCC_EXEC_PREFIX ?  If so, I'll propagate that
variable's value somehow to cobol.  If not, I'll roll my own.

Obviously I'd like to do that without stepping on the default
behavior.  Pointers welcome.  :-)

cobol1 needs this information to find include files, akin to
<algorithm> in C++.  

--jkl