Increase CMake version requirement to 3.6.3

LLVM 7.0 already demands 3.4.3, and 3.6 improved Ninja support.

Bug: b/139189696
Change-Id: Ic94b6828e686646198d08bdd9bd886caa56a51df
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31770
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 71385bf..5b498b3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.6.3)
 
 project(SwiftShader C CXX ASM)