Fix startx & xinitrc to handle values with spaces

To avoid having to add messy cpp magic to stringify arguments passed via #define, changes these files to be generated by configure substitutions instead of cpp pre-processing, bringing us a step closer to #15.

Tested on Solaris 11.4.

Closes: #23 (closed)
Reported-by: @ptribble
Fixes: b8517b64 ("Modernized shell scripts")