Increase minimum required cmake version

OBJECT library types are supported only in CMake 2.8.8 or higher, and
attempting to build BoringSSL on Ubuntu 12.04 results in CMake
displaying unhelpful error messages.

Change-Id: I2bc77a2c95d4f6ee41f8489ff679a2a0ba48c508
Reviewed-on: https://boringssl-review.googlesource.com/1530
Reviewed-by: Adam Langley <agl@google.com>

BUILDING

@@ -1,12 +1,8 @@
-cmake --version  # ensure that you have >= 2.8.8
-
 mkdir build
 cd build
 cmake ..
 make
 
-If you see an error about "Cannot find source file: OBJECT" then your version of CMake is too old.
-
 Note that the default build flags in the top-leve CMakeLists.txt are for
 debugging - optimisation isn't enabled.
 

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 2.8)
+cmake_minimum_required (VERSION 2.8.8)
 
 project (BoringSSL)