SystemTools: Use COMPILE_DEFINITIONS to pass platform tests

Replace use of COMPILE_FLAGS with COMPILE_DEFINITIONS for the
SystemTools.cxx source file property and spell out each definition on
its own line.

Change-Id: I213326b25fc7510107b78a8e233a3198862d9309

CMakeLists.txt

@@ -569,8 +569,11 @@ IF(KWSYS_USE_SystemTools)
     "Checking whether CXX compiler has unsetenv" DIRECT)
   KWSYS_PLATFORM_CXX_TEST(KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H
     "Checking whether CXX compiler has environ in stdlib.h" DIRECT)
-  SET_SOURCE_FILES_PROPERTIES(SystemTools.cxx PROPERTIES
-    COMPILE_FLAGS "-DKWSYS_CXX_HAS_SETENV=${KWSYS_CXX_HAS_SETENV} -DKWSYS_CXX_HAS_UNSETENV=${KWSYS_CXX_HAS_UNSETENV} -DKWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H=${KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H}")
+  SET_PROPERTY(SOURCE SystemTools.cxx APPEND PROPERTY COMPILE_DEFINITIONS
+    KWSYS_CXX_HAS_SETENV=${KWSYS_CXX_HAS_SETENV}
+    KWSYS_CXX_HAS_UNSETENV=${KWSYS_CXX_HAS_UNSETENV}
+    KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H=${KWSYS_CXX_HAS_ENVIRON_IN_STDLIB_H}
+    )
 ENDIF()
 
 IF(KWSYS_USE_SystemInformation)