ENH: Conditionally add testing for External Modules
ENH: Conditionally add testing for External Modules

Adds a check on the BUILD_TESTING variable before adding
the test subdirectory when building external modules against
an ITK build tree.
The include of Ctest before the 'if' statement ensures that
BUILD_TESTING is set.

Change-Id: Ib56d7e0586eb24a2377c520503ad633f22af9b8e

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