build: Bump minimum CMake to 3.14.

This is required since the install() cleanup in
e690fc21bb.

CMakeLists.txt

@@ -1,7 +1,7 @@
 # TODO
 # - backend selection via command line, rather than simply detecting headers.
 
-cmake_minimum_required(VERSION 3.7 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
 project(cubeb
   VERSION 0.0.0)