Fix make distclean to remove generated sources.mk

Make.mk

@@ -88,7 +88,14 @@ subst.in = sed \
 
 # Generated files
 # See `Cleanliness' below
-# sources.mk subjects.mk
+mk := subjects.mk
+ifeq ($(revctrl),git)
+mk += sources.mk
+else
+ifneq ($(srcdir),.)
+mk += sources.mk
+endif
+endif
 # Generated by Autoconf, not distributed:
 ac := config.h config.log config.status info.html info.nr lib stamp-h
 ac += $(basename $(filter %.in, $(src)))
@@ -138,7 +145,7 @@ endif
 clean := $(obj) $(deps) $(libs) $(util) $(client) $(server) $(tsubj)	\
 $(ttop) $(info.nr) $(info.html) $(empth_obj) $(empth_lib)
 # Removed by distclean:
-distclean := $(ac) subjects.mk
+distclean := $(ac) $(mk)
 # Distributed by dist-source from $(srcdir)
 src_distgen := $(acdist)
 # Distributed by dist-source from .
@@ -323,7 +330,8 @@ info.ps: info/TROFF.MAC info/INFO.MAC $(ttop) $(tsubj) $(tsrc)
 
 # Note: $(srcdir)/sources.mk is only used when the source tree came
 # from a tarball rather than git.  The following rules create a
-# sources.mk to put into the tarball.  It is not used in this build.
+# sources.mk to put into the tarball.  It is not used otherwise in
+# this build.
 
 ifeq ($(revctrl),git)
 sources.mk: