Update DEPEND_CMD comment to better explain why we postprocess gcc output.

Originally committed as revision 15628 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1012,7 +1012,8 @@ SLIBNAME_WITH_VERSION='$(SLIBNAME).$(LIBVERSION)'
 SLIBNAME_WITH_MAJOR='$(SLIBNAME).$(LIBMAJOR)'
 LIB_INSTALL_EXTRA_CMD='$(RANLIB) "$(LIBDIR)/$(LIBNAME)"'
 
-# gcc stupidly only outputs the basename of targets with -MM
+# gcc stupidly only outputs the basename of targets with -MM, but we need the
+# full relative path for objects in subdirectories for non-recursive Make.
 DEPEND_CMD='$(CC) $(CFLAGS) -MM -MG $< | sed -e "/^\#.*/d" -e "1s,^,$(@D)/," -e "s,\\([[:space:]]\\)\\(version\\.h\\),\\1\$$(BUILD_ROOT_REL)/\\2,"'
 
 # find source path