Move check for non-standard Eigen installs into FindEigen.cmake

- As part of adding support for detection of non-standard Homebrew
  install on OSX, an Eigen specific search directory was added to the
  search path in the main CMakeLists.  This commit moves that search
  into FindEigen.cmake.

Change-Id: Ia6142211961a0d1fdddeea77b83438e4ef94434f
2 files changed