Merge #140: build: don't clobber user/superproject c++ version

df21535 build: don't clobber user/superproject c++ version (Cory Fields)

Pull request description:

  This allows a vendored libmultiprocess to inherit the c++ version from the main project

ACKs for top commit:
  hebasto:
    ACK df21535.
  ryanofsky:
    Code review ACK df21535

Tree-SHA512: 700a2b04cd704131e2a38e07a9a5466b52487845b3c63b4a615949a730a560176ae8daf2d461c48279a89545e6a973a6fc97b97a1b6c0bd8e164885cb848cd8f

Author: ryanofsky

CMakeLists.txt

@@ -5,8 +5,10 @@
 cmake_minimum_required(VERSION 3.8)
 
 project("Libmultiprocess" CXX)
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED YES)
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED YES)
+endif()
 
 include("cmake/compat_find.cmake")