Add pthread to static dependencies

Change-Id: Ibd1e82c276c84de2bda03bf979870b081f6b9147

cpp/CMakeLists.txt

@@ -971,6 +971,7 @@ if (ARROW_JEMALLOC)
         # For glibc <2.17 we need to link to librt.
         # As we compile with --as-needed by default, the linker will omit this
         # dependency if not required.
+        ${PTHREAD_LIBRARY}
         rt
         )
     else()