The fptools suite consists of several projects, most of which can be downloaded, built and installed individually. Each project corresponds to a subdirectory in the source tree, and if checking out from CVS then each project can be checked out individually by sitting in the top level of your source tree and typing cvs checkout project.
Here is a list of the projects currently available:
The Glasgow Haskell Compiler (minus libraries). Absolutely required for building GHC.
Utility programs, some of which are used by the build/installation system. Required for pretty much everything.
The Green Card system for generating Haskell foreign function interfaces.
The Haggis Haskell GUI framework.
The Haddock documentation tool.
The Happy Parser generator.
The H/Direct Haskell interoperability tool.
Supplemental libraries for GHC (required for building GHC).
Hierarchical Haskell library suite (required for building GHC).
The Modular Haskell Metric System.
The NoFib suite: A collection of Haskell programs used primarily for benchmarking.
A testing framework, including GHC's regression test suite.
So, to build GHC you need at least the ghc, libraries and hslibs projects (a GHC source distribution will already include the bits you need).