cannot configure with bison-2.7

[fangism]

When YACC=bison (as opposed to yacc that is bison -y):

checking for bison... /usr/local/experimental/bin/bison
clang: error: no such file or directory: 'y.tab.c'
clang: error: no input files
grep: y.tab.c: No such file or directory
grep: y.tab.c: No such file or directory
configure: parser generator is really yacc
grep: y.tab.c: No such file or directory
configure: error: unable to determine /usr/local/experimental/bin/bison version!

This is because the m4 autoconf tests check for y.tab.c, when bison produces conftest.tab.c (or whatever the base name of the yacc file is).

e.g. fails with bison-2.7 and probably many other versions.