* configure.ac: AC_PREREQ 2.61, for AC_PROG_MKDIR_P.

* config.make.in (datarootdir): New directory variable (for
autoconf-2.61).

Rev: src/nettle/ChangeLog:1.403
Rev: src/nettle/config.make.in:1.12
Rev: src/nettle/configure.ac:1.81