diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2a4b400..e5f2ad9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -88,28 +88,34 @@
 endif()
 set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS Debug Release MinSizeRel RelWithDebInfo)
 
-option(SWIFTSHADER_BUILD_EGL "Build the EGL library" 1)
-option(SWIFTSHADER_BUILD_GLESv2 "Build the OpenGL ES 2 library" 1)
-option(SWIFTSHADER_BUILD_GLES_CM "Build the OpenGL ES 1.1 library" 1)
-option(SWIFTSHADER_BUILD_VULKAN "Build the Vulkan library" 1)
-option(SWIFTSHADER_BUILD_PVR "Fetch and build the PowerVR examples" 0)
+function (option_if_not_defined name description default)
+    if(NOT DEFINED ${name})
+        option(${name} ${description} ${default})
+    endif()
+endfunction()
 
-option(SWIFTSHADER_USE_GROUP_SOURCES "Group the source files in a folder tree for Visual Studio" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_EGL "Build the EGL library" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_GLESv2 "Build the OpenGL ES 2 library" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_GLES_CM "Build the OpenGL ES 1.1 library" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_VULKAN "Build the Vulkan library" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_PVR "Fetch and build the PowerVR examples" 0)
 
-option(SWIFTSHADER_BUILD_SAMPLES "Build sample programs" 1)
-option(SWIFTSHADER_BUILD_TESTS "Build test programs" 1)
+option_if_not_defined(SWIFTSHADER_USE_GROUP_SOURCES "Group the source files in a folder tree for Visual Studio" 1)
 
-option(SWIFTSHADER_MSAN "Build with memory sanitizer" 0)
-option(SWIFTSHADER_ASAN "Build with address sanitizer" 0)
-option(SWIFTSHADER_TSAN "Build with thread sanitizer" 0)
-option(SWIFTSHADER_UBSAN "Build with undefined behavior sanitizer" 0)
-option(SWIFTSHADER_WARNINGS_AS_ERRORS "Treat all warnings as errors" 1)
-option(SWIFTSHADER_DCHECK_ALWAYS_ON "Check validation macros even in release builds" 0)
-option(REACTOR_EMIT_DEBUG_INFO "Emit debug info for JIT functions" 0)
-option(REACTOR_EMIT_PRINT_LOCATION "Emit printing of location info for JIT functions" 0)
-option(REACTOR_VERIFY_LLVM_IR "Check reactor-generated LLVM IR is valid even in release builds" 0)
-option(SWIFTSHADER_LESS_DEBUG_INFO "Generate less debug info to reduce file size" 0)
-option(SWIFTSHADER_ENABLE_VULKAN_DEBUGGER "Enable vulkan debugger support" 0)
+option_if_not_defined(SWIFTSHADER_BUILD_SAMPLES "Build sample programs" 1)
+option_if_not_defined(SWIFTSHADER_BUILD_TESTS "Build test programs" 1)
+
+option_if_not_defined(SWIFTSHADER_MSAN "Build with memory sanitizer" 0)
+option_if_not_defined(SWIFTSHADER_ASAN "Build with address sanitizer" 0)
+option_if_not_defined(SWIFTSHADER_TSAN "Build with thread sanitizer" 0)
+option_if_not_defined(SWIFTSHADER_UBSAN "Build with undefined behavior sanitizer" 0)
+option_if_not_defined(SWIFTSHADER_WARNINGS_AS_ERRORS "Treat all warnings as errors" 1)
+option_if_not_defined(SWIFTSHADER_DCHECK_ALWAYS_ON "Check validation macros even in release builds" 0)
+option_if_not_defined(REACTOR_EMIT_DEBUG_INFO "Emit debug info for JIT functions" 0)
+option_if_not_defined(REACTOR_EMIT_PRINT_LOCATION "Emit printing of location info for JIT functions" 0)
+option_if_not_defined(REACTOR_VERIFY_LLVM_IR "Check reactor-generated LLVM IR is valid even in release builds" 0)
+option_if_not_defined(SWIFTSHADER_LESS_DEBUG_INFO "Generate less debug info to reduce file size" 0)
+option_if_not_defined(SWIFTSHADER_ENABLE_VULKAN_DEBUGGER "Enable vulkan debugger support" 0)
 
 set(BUILD_MARL ${SWIFTSHADER_BUILD_VULKAN})