Makefile's standard GHC options to @mkdependHS@ un-filtered.
<descrip>
+<tag>@-cpp@</tag> Run the C pre-processor over the input files. The
+ default is not to.
<tag>@-D<blah>@</tag> A cpp @#define@; usual meaning.
<tag>@-i<dirs>@</tag> Add @<dirs>@ (colon-separated) to list of directories