FindBlosc.cmake: Threads

Blosc depends unconditionally on threads.
If we are not using the config package, we need to re-add the
pthreads flags or we will get linker errors.

We are sometimes lucky at the moment that in some build configs
pthread flags land in the right order on the commands line, but
generally we should declare this dependency.

cmake/FindBlosc.cmake

@@ -54,6 +54,24 @@ if(NOT BLOSC_FOUND)
     endif()
   endif()
 
+  # Blosc depends on pthreads
+  # https://github.com/Blosc/c-blosc/blob/master/blosc/CMakeLists.txt
+  # https://github.com/Blosc/c-blosc/pull/318
+  set(THREADS_PREFER_PTHREAD_FLAG TRUE)
+  if(WIN32)
+    # try to use the system library
+    find_package(Threads)
+    if(NOT Threads_FOUND)
+      message(STATUS "Blosc: used the internal pthread library for win32 systems.")
+      set(BLOSC_LIBRARIES)
+    else()
+      set(BLOSC_LIBRARIES Threads::Threads)
+    endif()
+  else()
+    find_package(Threads REQUIRED)
+    set(BLOSC_LIBRARIES Threads::Threads)
+  endif()
+
   include(FindPackageHandleStandardArgs)
   find_package_handle_standard_args(Blosc
     FOUND_VAR BLOSC_FOUND
@@ -68,6 +86,7 @@ if(NOT BLOSC_FOUND)
       set_target_properties(Blosc::Blosc PROPERTIES
         IMPORTED_LOCATION             "${BLOSC_LIBRARY}"
         INTERFACE_INCLUDE_DIRECTORIES "${BLOSC_INCLUDE_DIR}"
+        INTERFACE_LINK_LIBRARIES      "${BLOSC_LIBRARIES}"
       )
     endif()
   endif()