Tests: Compile entire Plugin test with the same language standard

Set `CMAKE_CXX_STANDARD` early so that both KWSys and our test code
build with the same language standard.  This is important on compilers
that have incompatible standard libraries.

Tests/Plugin/CMakeLists.txt

@@ -2,6 +2,9 @@ cmake_minimum_required (VERSION 2.6)
 cmake_policy(SET CMP0054 NEW)
 project(Plugin)
 
+# We need proper C++98 support from the compiler
+set(CMAKE_CXX_STANDARD 98)
+
 # Test per-target output directory properties.
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${Plugin_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${Plugin_BINARY_DIR}/lib/plugin)
@@ -25,9 +28,6 @@ include_directories(
   ${Plugin_SOURCE_DIR}/include
   )
 
-# We need proper C++98 support from the compiler
-set(CMAKE_CXX_STANDARD 98)
-
 # Those versions of the HP compiler that need a flag to get proper C++98
 # template support also need a flag to use the newer C++ library.
 if (CMAKE_CXX_COMPILER_ID STREQUAL HP AND