Make CMake util aom_push_var additive.

Change-Id: I949558a3fc716e717e3d0839a6a1ae4684ca7125
diff --git a/build/cmake/compiler_tests.cmake b/build/cmake/compiler_tests.cmake
index c759234..8a73ec8 100644
--- a/build/cmake/compiler_tests.cmake
+++ b/build/cmake/compiler_tests.cmake
@@ -53,8 +53,8 @@
 set(AOM_CXX_FAILED_TESTS)
 
 function(aom_push_var var new_value)
-  set(SAVED_${var} ${var} PARENT_SCOPE)
-  set(${var} ${new_value} PARENT_SCOPE)
+  set(SAVED_${var} ${${var}} PARENT_SCOPE)
+  set(${var} "${${var}} ${new_value}" PARENT_SCOPE)
 endfunction ()
 
 function(aom_pop_var var)