Support 'preprocessing only' and 'unifdef only'

Introduce two new pattern:
1) %.c.i: preprocess %.c into %.c.i using gcc -E with CPPFLAGS
2) %.c.u: preprocess #ifdef'd sections in %.c into %.c.u (so that results of
   #ifdef configuration et al can be seen) using unifdef with CPPFLAGS

Both extensions are not that well chosen because vim doesn't recognise
either one by default... There are also "#" lines from cpp in the %.c.i
files, sigh.