add libflint-dev to install instructions for debian, ubuntu

M2/INSTALL

@@ -83,7 +83,7 @@ various modern operating systems:
 
   Ubuntu and Debian:
     Install packages as root with:
-      apt-get install -y -q sudo 4ti2 autoconf bison curl emacs fflas-ffpack flex g++ gcc gfortran install-info libatomic-ops-dev libboost-dev libboost-regex-dev libboost-stacktrace-dev libc6-dev libcdd-dev libgc-dev libgdbm-dev libgivaro-dev libglpk-dev libgmp3-dev libgtest-dev liblapack-dev liblzma-dev libmathic-dev libmathicgb-dev libmemtailor-dev libmpc-dev libmpfr-dev libncurses-dev libncurses5-dev libntl-dev libreadline-dev libsingular-dev libtool libxml2-dev libz-dev lrslib lsb-release make normaliz openssh-server patch pinentry-curses pkg-config singular-data time unzip xbase-clients yasm zlib1g-dev polymake w3c-markup-validator git dpkg-dev gfan libeigen3-dev libtool-bin frobby libfrobby-dev libnauty2-dev libnauty2 nauty nauty-doc coinor-csdp coinor-csdp-doc
+      apt-get install -y -q sudo 4ti2 autoconf bison curl emacs fflas-ffpack flex g++ gcc gfortran install-info libatomic-ops-dev libboost-dev libboost-regex-dev libboost-stacktrace-dev libc6-dev libcdd-dev libgc-dev libgdbm-dev libgivaro-dev libglpk-dev libgmp3-dev libgtest-dev liblapack-dev liblzma-dev libmathic-dev libmathicgb-dev libmemtailor-dev libmpc-dev libmpfr-dev libncurses-dev libncurses5-dev libntl-dev libreadline-dev libsingular-dev libtool libxml2-dev libz-dev lrslib lsb-release make normaliz openssh-server patch pinentry-curses pkg-config singular-data time unzip xbase-clients yasm zlib1g-dev polymake w3c-markup-validator git dpkg-dev gfan libeigen3-dev libtool-bin frobby libfrobby-dev libnauty2-dev libnauty2 nauty nauty-doc coinor-csdp coinor-csdp-doc libflint-dev
         # note: libz-dev seems to have been replaced by zlib1g-dev
         # note: libncurses-dev seems to have been replaced by libncurses5-dev
         # note: libreadline-gplv2-dev is an older GPL v2 version of libreadline