ENH: Teach ITK module macros to support independent header-only modules
ENH: Teach ITK module macros to support independent header-only modules

Allow an empty <mod>_LIBRARIES list, so long as the variable is defined.
This is needed to allow header-only modules with no dependencies on
modules with actual libraries.

Change-Id: Ic2ea8620ff599c000fadaa54688cb0ead622f52f

vfonov/ITK,LucasGandel/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,malaterre/ITK,malaterre/ITK,zachary-williamson/ITK,hjmjohnson/ITK,hjmjohnson/ITK,spinicist/ITK,blowekamp/ITK,fbudin69500/ITK,stnava/ITK,blowekamp/ITK,richardbeare/ITK,LucasGandel/ITK,LucasGandel/ITK,fbudin69500/ITK,LucasGandel/ITK,richardbeare/ITK,fbudin69500/ITK,Kitware/ITK,stnava/ITK,zachary-williamson/ITK,thewtex/ITK,spinicist/ITK,zachary-williamson/ITK,stnava/ITK,hjmjohnson/ITK,spinicist/ITK,BRAINSia/ITK,hjmjohnson/ITK,spinicist/ITK,jcfr/ITK,PlutoniumHeart/ITK,Kitware/ITK,spinicist/ITK,richardbeare/ITK,jcfr/ITK,Kitware/ITK,jcfr/ITK,zachary-williamson/ITK,malaterre/ITK,stnava/ITK,Kitware/ITK,hjmjohnson/ITK,vfonov/ITK,BRAINSia/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,PlutoniumHeart/ITK,BRAINSia/ITK,vfonov/ITK,zachary-williamson/ITK,PlutoniumHeart/ITK,blowekamp/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,LucasGandel/ITK,jcfr/ITK,fbudin69500/ITK,fbudin69500/ITK,jcfr/ITK,vfonov/ITK,zachary-williamson/ITK,stnava/ITK,malaterre/ITK,spinicist/ITK,BRAINSia/ITK,zachary-williamson/ITK,vfonov/ITK,PlutoniumHeart/ITK,fbudin69500/ITK,vfonov/ITK,PlutoniumHeart/ITK,stnava/ITK,blowekamp/ITK,blowekamp/ITK,vfonov/ITK,richardbeare/ITK,jcfr/ITK,thewtex/ITK,malaterre/ITK,jcfr/ITK,spinicist/ITK,BRAINSia/ITK,spinicist/ITK,thewtex/ITK,blowekamp/ITK,blowekamp/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,PlutoniumHeart/ITK,vfonov/ITK,Kitware/ITK,stnava/ITK,PlutoniumHeart/ITK,LucasGandel/ITK,blowekamp/ITK,BRAINSia/ITK,stnava/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,spinicist/ITK,richardbeare/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,LucasGandel/ITK,malaterre/ITK,BRAINSia/ITK,PlutoniumHeart/ITK,zachary-williamson/ITK,fbudin69500/ITK,richardbeare/ITK,zachary-williamson/ITK,jcfr/ITK,InsightSoftwareConsortium/ITK,fbudin69500/ITK,richardbeare/ITK,Kitware/ITK