Implemented a method to handle empty subcells that exist because a

library has been read in with the "gds readonly true" option set
because the cell contains information on where in the GDS the
cell is located, but the cell is empty because it was flattened
into the magic view and all of its contents were erased.  This can
cause issues with LVS if magic generates an empty cell into the
netlist and the LVS tool tries to compare the cells by name.  Also,
this prevents unnecessary .ext files and unnecessary merges to the
substrate of such cells (since all cells have an implied substrate).

database/database.h.in

@@ -418,6 +418,8 @@ typedef struct celldef
  *	    the same timestamp value.
  *	CDNOEXTRACT is used by incremental extraction to flag cells that
  *	    are up-to-date and do not need to be re-extracted.
+ *	CDDONTUSE is used during extraction to flag cells that have no
+ *	    contents and should be ignored.
  */
 
 #define	CDAVAILABLE	 0x00001
@@ -438,6 +440,7 @@ typedef struct celldef
 #define CDDEREFERENCE	 0x08000
 #define CDFIXEDSTAMP	 0x10000
 #define CDNOEXTRACT	 0x20000
+#define CDDONTUSE	 0x40000
 
 /*
  * Description of an array.