This makes cross-compiling easier, and saves having to build once each
for TFE and TSE.
I considered using CMake's import/export feature, but there's little
point when you're only importing one binary target. Pointing the ECC
variable at the ecc binary itself is simpler.
I copied the minimum CMake version of 2.8.7 from the parent
project. To be confident this would actually work, I tested building
under CentOS 7 with CMake 2.8.12.
c09d18623e