Add /Zm flag as a workaround for VS2013 build (#2023)

diff --git a/source/opt/CMakeLists.txt b/source/opt/CMakeLists.txt
index 254a4ae..fd52ca6 100644
--- a/source/opt/CMakeLists.txt
+++ b/source/opt/CMakeLists.txt
@@ -195,8 +195,8 @@
   else()
     set(PCH_NAME "pch.pch")
   endif()
-  set_source_files_properties(${SPIRV_TOOLS_OPT_SOURCES} PROPERTIES COMPILE_FLAGS "/Yupch.h /FIpch.h /Fp${PCH_NAME}" OBJECT_DEPENDS "${PCH_NAME}")
-  set_source_files_properties(pch.cpp PROPERTIES COMPILE_FLAGS "/Ycpch.h /Fp${PCH_NAME}" OBJECT_OUTPUTS "${PCH_NAME}")
+  set_source_files_properties(${SPIRV_TOOLS_OPT_SOURCES} PROPERTIES COMPILE_FLAGS "/Yupch.h /FIpch.h /Fp${PCH_NAME} /Zm300" OBJECT_DEPENDS "${PCH_NAME}")
+  set_source_files_properties(pch.cpp PROPERTIES COMPILE_FLAGS "/Ycpch.h /Fp${PCH_NAME} /Zm300" OBJECT_OUTPUTS "${PCH_NAME}")
   list(APPEND SPIRV_TOOLS_OPT_SOURCES "pch.cpp")
 endif()