Make colour macros more flexible.

Instead of COLOUR0_PRIMARY et al. expanding directly to X11 colour
names, allow the process to be hooked to generate other forms of
colour definitions.

The ewmhicon test program is adapted to use this to generate its
colourmap.  This also avoids some apparently nonportable C macro
usage, as preprocessing the following code:

  #define A(x) x
  #define B(x) x
  #define C (x)
  A(B C)

expands to x (the desired result) with GCC but B(x) on HP-UX cc.