COMP: Add CMAKE_SH definition for Eigen ExternalProject
COMP: Add CMAKE_SH definition for Eigen ExternalProject

Required for MinGW-w64 builds, important for Windows and R.

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