I know the INSTALL file is missing from 4.3 so I can't check it directly, so while building a new copy of 4.3 from source I relied on the version from 4.2.1. It says:
1. `cd' to the directory containing the package's source code and type
`./configure' to configure the package for your system. If you're
using `csh' on an old version of System V, you might need to type
`sh ./configure' instead to prevent `csh' from trying to execute
`configure' itself.
Running `configure' takes awhile. While running, it prints some
messages telling which features it is checking for.
2. Type `make' to compile the package.
The first sentence "cd to the directory containing the package's source code" seems somewhere between misleading and just wrong. The directory containing source code is ./src but the configure script, and the subsequently generated Makefile, are both at the top level.
David