[CMake] Bump CMake minimum version to 3.13.4

This upgrade should be friction-less because we've already been ensuring
that CMake >= 3.13.4 is used.

This is part of the effort discussed on llvm-dev here:

  http://lists.llvm.org/pipermail/llvm-dev/2020-April/140578.html

Differential Revision: https://reviews.llvm.org/D78648

GitOrigin-RevId: afa1afd4108d973e059e5f5ad68cf01efe7985da

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6f0d9e7..f18688f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.13.4)
 
 # Add cmake directory to search for custom cmake functions.
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})

cmake/DetectTestCompiler/CMakeLists.txt

diff --git a/cmake/DetectTestCompiler/CMakeLists.txt b/cmake/DetectTestCompiler/CMakeLists.txt
index c230fc8..7fa32a9 100644
--- a/cmake/DetectTestCompiler/CMakeLists.txt
+++ b/cmake/DetectTestCompiler/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.13.4)
 project(DetectTestCompiler C CXX)
 
 include(CheckCCompilerFlag)

runtime/cmake/LibompCheckLinkerFlag.cmake

diff --git a/runtime/cmake/LibompCheckLinkerFlag.cmake b/runtime/cmake/LibompCheckLinkerFlag.cmake
index 2b6cda6..e601e53 100644
--- a/runtime/cmake/LibompCheckLinkerFlag.cmake
+++ b/runtime/cmake/LibompCheckLinkerFlag.cmake
@@ -17,7 +17,7 @@
   set(library_source
     "int foo(int a) { return a*a; }")
   set(cmake_source
-    "cmake_minimum_required(VERSION 2.8)
+    "cmake_minimum_required(VERSION 3.13.4)
      project(foo C)
      set(CMAKE_SHARED_LINKER_FLAGS \"${flag}\")
      add_library(foo SHARED src_to_link.c)")