Have consistent compile lines between BUILD_TESTS enabled or not (rap…

…idsai#1401)

This will remove 1h from our conda CI builds since we can now re-use the cached object files between `libraft` and `libraft-tests`

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Ben Frederickson (https://github.com/benfred)
  - Divye Gala (https://github.com/divyegala)

URL: rapidsai#1401

cpp/CMakeLists.txt

@@ -81,15 +81,12 @@ option(RAFT_COMPILE_LIBRARY "Enable building raft shared library instantiations"
        ${RAFT_COMPILE_LIBRARY_DEFAULT}
 )
 
-if(BUILD_TESTS
-   OR BUILD_PRIMS_BENCH
-   OR BUILD_ANN_BENCH
-)
-  # Needed because GoogleBenchmark changes the state of FindThreads.cmake, causing subsequent runs
-  # to have different values for the `Threads::Threads` target. Setting this flag ensures
-  # `Threads::Threads` is the same value in first run and subsequent runs.
-  set(THREADS_PREFER_PTHREAD_FLAG ON)
-endif()
+
+# Needed because GoogleBenchmark changes the state of FindThreads.cmake, causing subsequent runs
+# to have different values for the `Threads::Threads` target. Setting this flag ensures
+# `Threads::Threads` is the same value across all builds so that cache hits occur
+set(THREADS_PREFER_PTHREAD_FLAG ON)
+
 
 include(CMakeDependentOption)
 # cmake_dependent_option( RAFT_USE_FAISS_STATIC "Build and statically link the FAISS library for