odd bootstrap fail on Solaris (3.1 branch): bad Makefile generated

[Joe Buck <Joe dot Buck at synopsys dot com>]

I can't boot the 3.1 branch on sparc-sun-solaris2.7 without editing a
Makefile.  The only configure flag I give is --prefix, and I try to build
in a separate directory as usual.

The problem appears to be in the generated $build_dir/gcc/intl/Makefile .
It has among other things

LIBTOOL = @LIBTOOL@
YACC = : -y -d
YFLAGS = --name-prefix=__gettext

When I do a make bootstrap I get the following failure:

: -y -d --name-prefix=__gettext --output plural.c ../../../gcc/gcc/intl/plural.y
rm -f plural.h
gcc -c -DIN_GCC -DLOCALEDIR=\"/u/jbuck/gcc-3.1-pre.sol2/share/locale\" -DLOCALE_ALIAS_PATH=\"/u/jbuck/gcc-3.1-pre.sol2/share/locale\" -DLIBDIR=\"/u/jbuck/gcc-3.1-pre.sol2/lib\" -DHAVE_CONFIG_H -I.. -I. -I../../../gcc/gcc/intl -I../../../gcc/gcc -I../../../gcc/gcc/config -I../../../gcc/gcc/../include  -g  -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wtraditional -pedantic -Wno-long-long  plural.c -o plural.o
gcc: plural.c: No such file or directory
gcc: No input files
make[3]: *** [plural.o] Error 1
make[3]: Leaving directory `.../gcc.sol2/gcc/intl'

The @-signs are worrying, and the YACC variable is wrong, of course.  If I
replace the colon by "bison", then things seem to work OK (at least so
far, the build has not yet completed).  But why do we get the colon?
bison is successfully detected by every other GNU tool I configure on this
box.