CMake: No target "gie"

[mwtoews]

When configuring a CMake build of PROJ with:

cmake -DBUILD_GIE=OFF -DBUILD_TESTING=ON ..

an error is repeated over 30 times:

CMake Error at cmake/ProjTest.cmake:31 (add_test):
  Error evaluating generator expression:

    $<TARGET_FILE_NAME:gie>

  No target "gie"
Call Stack (most recent call first):
  test/CMakeLists.txt:6 (proj_add_gie_test)

Some options are:

1. Skip tests that require gie
2. Build gie so it can be used for tests, but don't install it

Unless anyone has an objection, I'll go for option 2.