D: Fix typos in RelWithDebInfo configuration
D: Fill in more compiler information for D

These variables are used to tell generators how to build up a command
line to use. Also fills in preprocessor and assembly commands (where
applicable).
D: Add -fPIE support for GDC
D: Fix static library compilation
DigitalMars-D: Spell 'WIN32' properly
D: Separate flags CMake understands from UseD variables

These variables are not standard CMake variables and should be separated
out.
Merge pull request #1 from mathstuf/d_support

D support