By default Debian dh passes --disable-dependacy-tracking when building.
If the package rules also adds --builddirectory to use out of source builds
(eg. "dh $@ --builddirectory=build --parallel --with autoreconf"), and the
package happens to be using subdir-objects automake option and generated source
files from lex/yacc then the build will fail.

Eg.

mkdir build
../configure --disable-dependency-tracking --disable-silent-rules
make

/bin/bash ../ylwrap ../src/scanner.ll lex.yy.c src/scanner.cc -- flex
../ylwrap: line 206: ../src/scanner.cc: No such file or directory
Makefile:422: recipe for target 'src/scanner.cc' failed
make: *** [src/scanner.cc] Error 1

A simple patch to lex.am and yacc.am fixes this.