COMP: Disable GTest using threads
COMP: Disable GTest using threads

Removes Threads::Threads form exported libraries dependencies, to
simply exports.

Addresses #1502.

BRAINSia/ITK,richardbeare/ITK,thewtex/ITK,Kitware/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,LucasGandel/ITK,malaterre/ITK,BRAINSia/ITK,hjmjohnson/ITK,hjmjohnson/ITK,LucasGandel/ITK,Kitware/ITK,hjmjohnson/ITK,richardbeare/ITK,vfonov/ITK,hjmjohnson/ITK,Kitware/ITK,thewtex/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK,BRAINSia/ITK,malaterre/ITK,BRAINSia/ITK,vfonov/ITK,vfonov/ITK,malaterre/ITK,richardbeare/ITK,richardbeare/ITK,BRAINSia/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,thewtex/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,Kitware/ITK,LucasGandel/ITK,vfonov/ITK,LucasGandel/ITK,malaterre/ITK,malaterre/ITK,BRAINSia/ITK,LucasGandel/ITK,richardbeare/ITK,vfonov/ITK,thewtex/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,vfonov/ITK,vfonov/ITK,Kitware/ITK,thewtex/ITK,malaterre/ITK,LucasGandel/ITK,LucasGandel/ITK