Process: Refactor test compile flag addition

CMakeLists.txt

@@ -1147,17 +1147,19 @@ IF(KWSYS_STANDALONE OR CMake_SOURCE_DIR)
       SET_TESTS_PROPERTIES(kwsys.testProcess-${n} PROPERTIES TIMEOUT 120)
     ENDFOREACH()
 
+    SET(testProcess_COMPILE_FLAGS "")
     # Some Apple compilers produce bad optimizations in this source.
     IF(APPLE AND CMAKE_C_COMPILER_ID MATCHES "^(GNU|LLVM)$")
-      SET_SOURCE_FILES_PROPERTIES(testProcess.c PROPERTIES COMPILE_FLAGS -O0)
+      STRING(APPEND testProcess_COMPILE_FLAGS " -O0")
     ELSEIF(CMAKE_C_COMPILER_ID STREQUAL "XL" AND
            NOT (CMAKE_SYSTEM MATCHES "Linux.*ppc64le" AND
                 NOT CMAKE_C_COMPILER_VERSION VERSION_LESS "13.1.1"))
       # Tell IBM XL not to warn about our test infinite loop
       # v13.1.1 and newer on Linux ppc64le is clang based and does not accept
       # the -qsuppress option
-      SET_PROPERTY(SOURCE testProcess.c PROPERTY COMPILE_FLAGS -qsuppress=1500-010)
+      STRING(APPEND testProcess_COMPILE_FLAGS " -qsuppress=1500-010")
     ENDIF()
+    SET_PROPERTY(SOURCE testProcess.c PROPERTY COMPILE_FLAGS "${testProcess_COMPILE_FLAGS}")
 
     # Test SharedForward
     CONFIGURE_FILE(${PROJECT_SOURCE_DIR}/testSharedForward.c.in