Makefile-engine does not generate catkeys

[KapiX]

I have tried using make catkeys, but it keeps generating empty (30-something bytes) file. collectcatkeys issues warnings when used with -w and I have discovered that there is grep -av '^#' missing after gcc preprocessing step. Other targets use that too, so I assume this is an oversight.

Simple patch attached.

[waddlesplash]

Patch looks OK to me; if nobody has any comments, I'll merge it shortly.

[waddlesplash]

Applied in hrev50857. Thanks!