Merge #596: Make WINDOW_G configurable

a61a93f Clean up ./configure help strings (Tim Ruffing)
2842dc5 Make WINDOW_G configurable (Tim Ruffing)

Pull request description:

  This makes WINDOW_G a configurable value in the range of [2..24].
  The upper limit of 24 is a defensive choice. The code is probably
  correct for values up to 33 but those larger values yield in huge
  tables (>= 256MiB), which are i) unlikely to be really beneficial
  in practice and ii) increasingly difficult to test.

  The main point of this is not to make the window size configurable (using ./configure) but rather to use an external #define for the window size, which makes it configurable for embedded system that rely on their own build system (like in #595).

ACKs for commit a61a93:

Tree-SHA512: 0d58fdf4763340ddab992e95f6302a33d891476a7ac1748202ee99808e72b20754bb6935cbeaf0bb36077abaaff7d65f4848b1af64f1a0a5258239ba0d27020c