Increase robustness of make install target

Use install to install colordiff, then execute perl in place.

Respect DESTDIR inside the colordiff script itself.

Unconditionally install cdiff.

Remove -D flag from install and manually create the MAN_DIR beforehand.
-D is not portable to Mac OS X.

Conditionally install man pages. This allows colordiff to be installed
on systems that are missing xmlto.

Unconditionally create the ETC_DIR.

Use install to install colordiffrc.