Bump the minimum CMake version to 3.12

CMake 3.12 was released July 2018, so it meets our five year rule. [0]
also requires a CMake version newer than 3.12.

[0] https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

Change-Id: I2b21d68e3a379108edde9c7d13450bba52f41235
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63105
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

BUILDING.md

@@ -10,7 +10,7 @@ Unless otherwise noted, build tools must at most five years old, matching
 [Abseil guidelines](https://abseil.io/about/compatibility). If in doubt, use the
 most recent stable version of each tool.
 
-  * [CMake](https://cmake.org/download/) 3.10 or later is required.
+  * [CMake](https://cmake.org/download/) 3.12 or later is required.
 
   * A recent version of Perl is required. On Windows,
     [Active State Perl](http://www.activestate.com/activeperl/) has been

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.10)
+cmake_minimum_required(VERSION 3.12)
 
 # Defer enabling C and CXX languages.
 project(BoringSSL NONE)

util/generate_build_files.py

@@ -417,7 +417,7 @@ class CMake(object):
     self.header = LicenseHeader("#") + R'''
 # This file is created by generate_build_files.py. Do not edit manually.
 
-cmake_minimum_required(VERSION 3.10)
+cmake_minimum_required(VERSION 3.12)
 
 project(BoringSSL LANGUAGES C CXX)