CMake: Warn if no build-type is selected.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.9)
 
 project(zlib
     LANGUAGES C
@@ -13,6 +13,13 @@ if(WIN32)
     option(ZLIB_INSTALL_COMPAT_DLL "Install a copy as zlib1.dll" ON)
 endif(WIN32)
 
+get_property(IS_MULTI GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+
+if(NOT DEFINED CMAKE_BUILD_TYPE AND NOT IS_MULTI)
+    message(STATUS "No CMAKE_BUILD_TYPE set -- using Release")
+    set(CMAKE_BUILD_TYPE Release)
+endif(NOT DEFINED CMAKE_BUILD_TYPE AND NOT IS_MULTI)
+
 include(CheckTypeSize)
 include(CheckFunctionExists)
 include(CheckIncludeFile)