bug#12899: Build failure from latest master

[Stefano Lattarini]

In a freshly-cloned repository on Debian with GNU make 3.82 and
gcc 4.7.2.:

  $ ./bootstrap && ./configure
  ... [all is ok]
  $ make all
    ...
    CC       src/echo.o
    CCLD     src/echo
    CC       src/env.o
    CCLD     src/env
    CC       src/expand.o
    CCLD     src/expand
    CC       src/expr.o
    CCLD     src/expr
    CC       src/factor.o
  src/factor.c:650:20: fatal error: primes.h: No such file or directory
  compilation terminated.
  make[2]: *** [src/factor.o] Error 1

Am I the only one encountering such failures?  If yes, I'll provide
more information about my environment in order to help debugging this.

Regards,
  Stefano