Add CMake build files

.gitignore

@@ -1,5 +1,6 @@
 .DS_Store
 /.build
+/out
 /Packages
 /*.xcodeproj
 Package.pins
@@ -13,3 +14,4 @@ DerivedData
 **/gyb.pyc
 
 .*.sw[nop]
+

CMakeLists.txt

@@ -0,0 +1,43 @@
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftCrypto open source project
+##
+## Copyright (c) 2021 Apple Inc. and the SwiftCrypto project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.md for the list of SwiftCrypto project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
+
+cmake_minimum_required(VERSION 3.15.1)
+
+project(SwiftCrypto
+  LANGUAGES ASM C Swift)
+
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
+
+include(SwiftSupport)
+
+if(CMAKE_VERSION VERSION_LESS 3.16 AND CMAKE_SYSTEM_NAME STREQUAL Windows)
+  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+else()
+  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+endif()
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
+
+if(CMAKE_SYSTEM_NAME STREQUAL Windows OR CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  option(BUILD_SHARED_LIBS "Build shared libraries by default" YES)
+endif()
+
+if(BUILD_SHARED_LIBS)
+  set(CMAKE_POSITION_INDEPENDENT_CODE YES)
+endif()
+
+add_subdirectory(Sources)
+add_subdirectory(cmake/modules)