CMake: change default ("") build type to Release (#818)

Fix #817
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2520278..9956a35 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -6,6 +6,13 @@
 
 project(brotli C)
 
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+  message(STATUS "Setting build type to Release as none was specified.")
+  set(CMAKE_BUILD_TYPE "Release" CACHE STRING "Choose the type of build." FORCE)
+else()
+  message(STATUS "Build type is '${CMAKE_BUILD_TYPE}'")
+endif()
+
 include(CheckCSourceCompiles)
 check_c_source_compiles(
   "#if defined(__EMSCRIPTEN__)