Building tip on freebsd, part II -- unrecognized '%' directive

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I have made some progress in building 
octave on Freebsd, using a liberal
sprinkling of envars, but I am now getting a new error when it
 tries (I think) to apply bison (my version is 2.4.3 fwiw).

Any thoughts?

My configure command line:

$ LDFLAGS="-L/usr/local/lib -L/usr/lib 
-L$HOME/SOURCES/octave/libgnu"
CPPFLAGS="-I/usr/local/include -I/usr/include
 -I$HOME/SOURCES/octave/libgnu"
YACC='bison' ./configure --prefix=/opt/octave-tip

My make command line (I don't know how they work, 
so I repeat the envars):

$ LDFLAGS="-L/usr/local/lib -L/usr/lib 
-L$HOME/SOURCES/octave/libgnu"
CPPFLAGS="-I/usr/local/include -I/usr/include 
-I$HOME/SOURCES/octave/libgnu" YACC='bison' gmake

The offending output from gmake:

Making all in src
gmake[2]: Entering directory 
`/usr/home/wsprague/SOURCES/octave/src'
/bin/sh ../build-aux/ylwrap lex.ll lex.octave_.c 
lex.cc -- flex -I -I
"/usr/home/wsprague/SOURCES/octave/src/lex.ll",
 line 27: unrecognized '%' directive
<SNIP>