If you're using IlmBase from github, you should run the bootstrap script 
to create the auto* files.  It's a good idea to run this whenever you
update IlmBase from github.

Then run './configure' and make.

Note that the configure.ac file requires a fairly new version of
automake.  If you get this error message:

running aclocal ...
aclocal: configure.ac: 142: macro `AM_CFLAGS' not found in library
aclocal: configure.ac: 143: macro `AM_CXXFLAGS' not found in library
failed!

you should upgrade your automake to version 1.6 or better.

