Don’t clobber CXX flags accidentally and make Release builds the default

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.11)
+cmake_minimum_required(VERSION 2.8.12)
 
 project(msdfgen)
 
@@ -8,7 +8,12 @@ include(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
 if (COMPILER_SUPPORTS_CXX11)
 	add_definitions(-DMSDFGEN_USE_CPP11)
-	set(CMAKE_CXX_FLAGS "-std=c++11")
+	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+endif()
+
+# Make release mode default (turn on optimizations)
+if(NOT CMAKE_BUILD_TYPE)
+  set(CMAKE_BUILD_TYPE Release)
 endif()