Lower cmake requirement for EL7
Hi,

Enterprise Linux 7 uses cmake 2.8, is it possible to lower the requirement to match that?

Tried to build like that, didn't encounter any problems.

```diff
--- a//CMakeLists.txt	2016-10-14 23:37:00.287093058 +0200
+++ b//CMakeLists.txt	2016-10-14 23:37:44.962803034 +0200
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 2.8)
 project(GLAD)
 
 set(GLAD_DIR "${PROJECT_SOURCE_DIR}")
```