ENH: Send an error when CMAKE_CXX_COMPILER_TARGET is not set.
ENH: Send an error when CMAKE_CXX_COMPILER_TARGET is not set.

CastXML needs this value to be set with the target triple when cross-compiling
the wrapping.  It is possible it could be not set if the cross-compiler is
GCC. Send an error at CMake time pointing the the Clang documentation on the
target triple.

Change-Id: Iad5731ee28bb4e7d526f1bbb3765b6ea0280bdcc

richardbeare/ITK,blowekamp/ITK,BlueBrain/ITK,msmolens/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,msmolens/ITK,malaterre/ITK,stnava/ITK,biotrump/ITK,fedral/ITK,spinicist/ITK,thewtex/ITK,PlutoniumHeart/ITK,BlueBrain/ITK,hjmjohnson/ITK,richardbeare/ITK,stnava/ITK,vfonov/ITK,BRAINSia/ITK,thewtex/ITK,zachary-williamson/ITK,blowekamp/ITK,biotrump/ITK,ajjl/ITK,fedral/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,msmolens/ITK,Kitware/ITK,msmolens/ITK,vfonov/ITK,stnava/ITK,biotrump/ITK,fedral/ITK,malaterre/ITK,stnava/ITK,biotrump/ITK,ajjl/ITK,PlutoniumHeart/ITK,fedral/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,hjmjohnson/ITK,BlueBrain/ITK,richardbeare/ITK,fbudin69500/ITK,zachary-williamson/ITK,PlutoniumHeart/ITK,jmerkow/ITK,jmerkow/ITK,ajjl/ITK,hjmjohnson/ITK,malaterre/ITK,fbudin69500/ITK,jmerkow/ITK,BlueBrain/ITK,Kitware/ITK,ajjl/ITK,LucHermitte/ITK,PlutoniumHeart/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,msmolens/ITK,jmerkow/ITK,stnava/ITK,jcfr/ITK,LucasGandel/ITK,LucasGandel/ITK,PlutoniumHeart/ITK,biotrump/ITK,blowekamp/ITK,vfonov/ITK,LucHermitte/ITK,BRAINSia/ITK,malaterre/ITK,vfonov/ITK,BRAINSia/ITK,zachary-williamson/ITK,vfonov/ITK,thewtex/ITK,blowekamp/ITK,richardbeare/ITK,malaterre/ITK,biotrump/ITK,BRAINSia/ITK,LucHermitte/ITK,LucHermitte/ITK,LucasGandel/ITK,biotrump/ITK,hjmjohnson/ITK,BlueBrain/ITK,Kitware/ITK,PlutoniumHeart/ITK,msmolens/ITK,msmolens/ITK,spinicist/ITK,jmerkow/ITK,jmerkow/ITK,thewtex/ITK,jmerkow/ITK,jcfr/ITK,vfonov/ITK,BlueBrain/ITK,fedral/ITK,spinicist/ITK,spinicist/ITK,stnava/ITK,fedral/ITK,hjmjohnson/ITK,spinicist/ITK,malaterre/ITK,LucasGandel/ITK,LucHermitte/ITK,spinicist/ITK,fbudin69500/ITK,Kitware/ITK,jcfr/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,BlueBrain/ITK,vfonov/ITK,biotrump/ITK,BRAINSia/ITK,spinicist/ITK,ajjl/ITK,blowekamp/ITK,BRAINSia/ITK,thewtex/ITK,spinicist/ITK,LucasGandel/ITK,fbudin69500/ITK,blowekamp/ITK,ajjl/ITK,jcfr/ITK,Kitware/ITK,blowekamp/ITK,fedral/ITK,malaterre/ITK,fbudin69500/ITK,zachary-williamson/ITK,jmerkow/ITK,fbudin69500/ITK,LucHermitte/ITK,LucHermitte/ITK,stnava/ITK,richardbeare/ITK,LucasGandel/ITK,ajjl/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,msmolens/ITK,malaterre/ITK,spinicist/ITK,stnava/ITK,zachary-williamson/ITK,stnava/ITK,fbudin69500/ITK,jcfr/ITK,fedral/ITK,Kitware/ITK,fbudin69500/ITK,richardbeare/ITK,vfonov/ITK,blowekamp/ITK,zachary-williamson/ITK,BlueBrain/ITK,LucHermitte/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,LucasGandel/ITK,PlutoniumHeart/ITK,thewtex/ITK,jcfr/ITK,jcfr/ITK,PlutoniumHeart/ITK,hjmjohnson/ITK,zachary-williamson/ITK,ajjl/ITK,zachary-williamson/ITK,jcfr/ITK,richardbeare/ITK