cmake: bump minimum CMake allowed to 3.11.0.

This cmake is 3 years old, but it removes confusion about the CFLAGS
environment variable (as of CMake 3.10.0, reference PR #4681) and also
consolidates resolves the need to have a separate requirement for Windows
Store apps (which requires CMake 3.11.0).

CMakeLists.txt

@@ -2,11 +2,10 @@ if(${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
   message(FATAL_ERROR "Prevented in-tree build. Please create a build directory outside of the SDL source code and run \"cmake -S ${CMAKE_SOURCE_DIR} -B .\" from there")
 endif()
 
-cmake_minimum_required(VERSION 3.0.0)
+cmake_minimum_required(VERSION 3.11.0)
 project(SDL2 C CXX)
 
 if(WINDOWS_STORE)
-  cmake_minimum_required(VERSION 3.11)
   add_definitions(-DSDL_BUILDING_WINRT=1 -ZW)
 endif()