Fix building on HP-UX 11.11 PA-RISC

By CMake 3.8.2 and GCC 4.7.1

build/cmake/CMakeLists.txt

@@ -115,9 +115,20 @@ endif ()
 # External dependencies
 #-----------------------------------------------------------------------------
 if (ZSTD_MULTITHREAD_SUPPORT AND UNIX)
-    set(THREADS_PREFER_PTHREAD_FLAG ON)
-    find_package(Threads REQUIRED)
-    if(CMAKE_USE_PTHREADS_INIT)
+    if (CMAKE_SYSTEM_NAME MATCHES "HP-UX")
+        find_package(Threads)
+        if (NOT Threads_FOUND)
+            set(CMAKE_USE_PTHREADS_INIT 1)
+            set(CMAKE_THREAD_LIBS_INIT -lpthread)
+            set(CMAKE_HAVE_THREADS_LIBRARY 1)
+            set(Threads_FOUND TRUE)
+        endif ()
+    else ()
+        set(THREADS_PREFER_PTHREAD_FLAG ON)
+        find_package(Threads REQUIRED)
+    endif ()
+
+    if (CMAKE_USE_PTHREADS_INIT)
         set(THREADS_LIBS "${CMAKE_THREAD_LIBS_INIT}")
     else()
         message(SEND_ERROR "ZSTD currently does not support thread libraries other than pthreads")