Update build instructions for Ubuntu/Debian (x64)

Added emacs, flex, and bison as dependencies. Removed libc6-dev and tcl-dev (implied by others), ia32-libs (lots of not needed libraries), and libgdbm-dev (wrong platform, and i386 version not easily available).

Removed line about building inside ./build, since that gave me file not found errors halfway through the build. Changed configure line to disable gdbm.