Debian GNU/Linux has

[Kalle Olavi Niemitalo]

lsh-snapshot-1998-12-28/bignum.h uses <gmp.h>, but that file doesn't
exist in Debian GNU/Linux.  Instead, there is <gmp2/gmp.h>.

I could write a configure test to detect this.  Should it add
-I/usr/include/gmp2 to CPPFLAGS or rename bignum.h to bignum.h.in and
process that with AC_SUBST(INCLUDE_GMP_H)?