How to change the dependencies a particular package has?

[Brian Buhrow <buhrow%nfbcal.org@localhost>]

	Hello.  For reasons beyond the scope of this message, I'm building
pkgsrc packages on an old FreeBSD-2.2.7 system.  I've run into a circular
dependency between flex and bison.  The native system has a sufficiently
current version of flex, so I'd like to remove the flex dependency from the
bison build process.    What I can't figure out is where that dependency is
referenced.  Below is the output of make which shows the problem.  

The Makefile for bison has:
USE_TOOLS+=             grep gm4:run msgfmt flex

But there are no references to a flex installation directory, a specific
version of flex that's required, or n options.mk file that contains said
information.  I've set:
TOOLS_PLATFORM.flex?=/usr/bin/flex
in /etc/mk.conf (/usr/pkg/etc/mk.conf in this case.)
Still, the system thinks it needs to build flex in order to build bison.  
I'm running a grep through the entire pkgsrc tree to find all references to
flex, but if anyone can shed light on how this dependency stuff works, that
would be very helpful.

-thanks
-Brian

P.S.   This is with CVS Tag: pkgsrc-2017Q1


=> Bootstrap dependency digest>=20010302: found digest-20160304
===> Skipping vulnerability checks.
=> Checksum SHA1 OK for bison-3.0.4.tar.xz
=> Checksum RMD160 OK for bison-3.0.4.tar.xz
=> Checksum SHA512 OK for bison-3.0.4.tar.xz
===> Installing dependencies for bison-3.0.4nb3
=> Tool dependency flex>=2.5.4a: NOT found
=> Verifying reinstall for ../../devel/flex
=> Bootstrap dependency digest>=20010302: found digest-20160304
===> Skipping vulnerability checks.
=> Checksum SHA1 OK for flex-2.6.3.tar.gz
=> Checksum RMD160 OK for flex-2.6.3.tar.gz
=> Checksum SHA512 OK for flex-2.6.3.tar.gz
===> Installing dependencies for flex-2.6.3nb1
=> Tool dependency libtool-base>=2.4.2nb9: found libtool-base-2.4.6
=> Tool dependency bash-[0-9]*: NOT found
=> Verifying reinstall for ../../shells/bash
=> Bootstrap dependency digest>=20010302: found digest-20160304
===> Skipping vulnerability checks.
=> Checksum SHA1 OK for bash-4.4.tar.gz
=> Checksum RMD160 OK for bash-4.4.tar.gz
=> Checksum SHA512 OK for bash-4.4.tar.gz
=> Checksum SHA1 OK for bash44-001
=> Checksum RMD160 OK for bash44-001
=> Checksum SHA512 OK for bash44-001
=> Checksum SHA1 OK for bash44-002
=> Checksum RMD160 OK for bash44-002
=> Checksum SHA512 OK for bash44-002
=> Checksum SHA1 OK for bash44-003
=> Checksum RMD160 OK for bash44-003
=> Checksum SHA512 OK for bash44-003
=> Checksum SHA1 OK for bash44-004
=> Checksum RMD160 OK for bash44-004
=> Checksum SHA512 OK for bash44-004
=> Checksum SHA1 OK for bash44-005
=> Checksum RMD160 OK for bash44-005
=> Checksum SHA512 OK for bash44-005
=> Checksum SHA1 OK for bash44-006
=> Checksum RMD160 OK for bash44-006
=> Checksum SHA512 OK for bash44-006
=> Checksum SHA1 OK for bash44-007
=> Checksum RMD160 OK for bash44-007
=> Checksum SHA512 OK for bash44-007
=> Checksum SHA1 OK for bash44-008
=> Checksum RMD160 OK for bash44-008
=> Checksum SHA512 OK for bash44-008
=> Checksum SHA1 OK for bash44-009
=> Checksum RMD160 OK for bash44-009
=> Checksum SHA512 OK for bash44-009
=> Checksum SHA1 OK for bash44-010
=> Checksum RMD160 OK for bash44-010
=> Checksum SHA512 OK for bash44-010
=> Checksum SHA1 OK for bash44-011
=> Checksum RMD160 OK for bash44-011
=> Checksum SHA512 OK for bash44-011
=> Checksum SHA1 OK for bash44-012
=> Checksum RMD160 OK for bash44-012
=> Checksum SHA512 OK for bash44-012
=> Tool dependency nbpatch-[0-9]*: found nbpatch-20151107
=> Build dependency gcc2>=2.95.3nb7: NOT found
=> Verifying reinstall for ../../lang/gcc2
=> Bootstrap dependency digest>=20010302: found digest-20160304
===> Skipping vulnerability checks.
=> Checksum SHA1 OK for gcc-2.95.3.tar.gz
=> Checksum RMD160 OK for gcc-2.95.3.tar.gz
=> Checksum SHA512 OK for gcc-2.95.3.tar.gz
=> Checksum SHA1 OK for gcc-2.95.3-diff-2002-08-29.gz
=> Checksum RMD160 OK for gcc-2.95.3-diff-2002-08-29.gz
=> Checksum SHA512 OK for gcc-2.95.3-diff-2002-08-29.gz
===> Installing dependencies for gcc2-2.95.3nb7
=> Tool dependency bison>=1.0: NOT found
=> Verifying reinstall for ../../devel/bison
ERROR: This package has set PKG_FAIL_REASON:
ERROR: Circular dependency detected
*** Error code 1