build: set minimum required CMake to 3.12

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -109,7 +109,7 @@
 # FreeType explicitly marks the API to be exported and relies on the compiler
 # to hide all other symbols. CMake supports a C_VISBILITY_PRESET property
 # starting with 2.8.12.
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.12)

 if (NOT CMAKE_VERSION VERSION_LESS 3.3)
   # Allow symbol visibility settings also on static libraries. CMake < 3.3