appveyor: turned off parallel build for clang, C++17

Author: trueqbit

appveyor.yml

@@ -46,7 +46,8 @@ environment:
       CC: clang
       CXX: clang++
       SQLITE_ORM_CXX_STANDARD: "-DSQLITE_ORM_ENABLE_CXX_17=ON"
-      cmake_build_parallel: --parallel
+      # clang was stuck with a parallel build
+      cmake_build_parallel: ""
 
     - job_name: gcc, C++17
       appveyor_build_worker_image: Ubuntu