[cmake] Drop the support of CMake 2.x.

* CMakeLists.txt: Require CMake 3.0 (released on 2014) or newer.

The issue #1059 reports the difficulty to support both of
CMake 2.x and newer one by single CMakeLists.txt without
the inflation of cmake_policy() workarounds.

For better maintainability, the support of CMake 2.x is
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 85babdd..9e66cde 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -106,10 +106,9 @@
 #   (this is compatible with the same CMake variables in zlib's CMake
 #   support).
-# 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)
+# To minimize the number of cmake_policy() workarounds,
+# CMake >= 3 is requested.
+cmake_minimum_required(VERSION 3.0)
   # Allow symbol visibility settings also on static libraries. CMake < 3.3