Change where thread support is detected to avoid using a new cmake fe…

…ature

Propagating library use to the global level is a newer cmake feature and
doesn't work reliably when included as a sub-project anyway. Remove this
and do the normal "find it at the top level" step inside the included
setup

Signed-off-by: Kimball Thurston <kdt3rd@gmail.com>

IlmBase/config/CMakeLists.txt

@@ -19,13 +19,8 @@ if(ILMBASE_HAVE_UCONTEXT_H)
   endif()
 endif()
 
-# so we know how to link / use...
-set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
-set(THREADS_PREFER_PTHREAD_FLAG TRUE)
-find_package(Threads)
 ###check_include_files(pthread.h ILMBASE_HAVE_PTHREAD)
 if(Threads_FOUND)
-  set_target_properties(Threads::Threads PROPERTIES IMPORTED_GLOBAL TRUE)
   if(CMAKE_HAVE_PTHREAD_H OR CMAKE_USE_PTHREADS_INIT OR CMAKE_HP_PTHREAD_INIT)
     set(ILMBASE_HAVE_PTHREAD ON)
     # TODO: remove this once we cleanly have ILMBASE_ prefix on all #defines

IlmBase/config/IlmBaseSetup.cmake

@@ -124,3 +124,17 @@ if(OPENEXR_USE_CLANG_TIDY)
     -checks=*;
   )
 endif()
+
+###############################
+# Dependent libraries
+
+# so we know how to link / use threads and don't have to have a -pthread
+# everywhere...
+if(NOT TARGET Threads::Threads)
+  set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
+  set(THREADS_PREFER_PTHREAD_FLAG TRUE)
+  find_package(Threads)
+  if(NOT Threads_FOUND)
+    message(FATAL_ERROR "Unable to find a threading library which is required for IlmThread")
+  endif()
+endif()