Add "libtool" to build dependencies.
Add "libtool" to build dependencies.

Our intern twice reported errors while setting up new machines for building autofdo.

ranlib libgflags.a
configure.ac:30: error: possibly undefined macro: AC_PROG_LIBTOOL
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf: /usr/bin/autoconf failed with exit status: 1

This was fixed by apt-get install libtool.

google/autofdo,google/autofdo,google/autofdo