Where do I get lib64 from?

[Temüjin]

flex is one of the build dependencies, and it should have been installed when you ran:

sudo apt-get build-dep grass

I assumed you ran that, but apparently, you didn't.... (silly me).

For reference, here's the list of stuff build-dep grass should install:

Build-depends: flex, bison,libreadline-dev | libreadline6-dev, libncurses5-dev, lesstif2-dev, debhelper (>= 7), 
 libtiff4-dev, tcl-dev (>= 8.5), tk-dev (>= 8.5), libfftw3-dev, libxmu-dev, libglu1-mesa-dev | libglu1-xorg-dev, 
 libfreetype6-dev, autoconf2.13, autotools-dev, libgdal1-dev (>= 1.5.0), libproj-dev, proj-bin, libjpeg-dev, 
 libpng12-dev, libpq-dev, unixodbc-dev, doxygen, fakeroot, libmysqlclient-dev, graphviz, libsqlite3-dev, python-wxgtk2.8,
 libcairo2-dev, libwxgtk2.8-dev, python-dev (>= 2.5), swig

[Pand5461]

I hadnt appreciated how tricky it could be to compile things on Linux.

It's easy if you try You might be interested in this howto.