build: add support to vendor dependencies

When building swift-crypto outside of the toolchain (e.g. to build LSP),
it is convenient to support vendoring dependencies.  Add support for
falling back to local builds of the dependencies when not provided as an
existing build.

CMakeLists.txt

@@ -12,6 +12,10 @@
 ##
 ##===----------------------------------------------------------------------===##
 
+if(POLICY CMP0077)
+  cmake_policy(SET CMP0077 NEW)
+endif()
+
 if(POLICY CMP0091)
   cmake_policy(SET CMP0091 NEW)
 endif()
@@ -37,11 +41,35 @@ set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
 set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
 set(CMAKE_POSITION_INDEPENDENT_CODE ${BUILD_SHARED_LIBS})
 
+# Toolchain vended dependencies
 find_package(dispatch CONFIG)
 find_package(Foundation CONFIG)
 
-find_package(SwiftASN1 CONFIG REQUIRED)
-find_package(SwiftCrypto CONFIG REQUIRED)
+include(FetchContent)
+
+set(_CERT_SAVED_BUILD_EXAMPLES ${BUILD_EXAMPLES})
+set(_CERT_SAVED_BUILD_TESTING ${BUILD_TESTING})
+
+find_package(SwiftASN1 CONFIG)
+if(NOT SwiftASN1_FOUND)
+  message("-- Vending swift-asn1")
+  FetchContent_Declare(ASN1
+    GIT_REPOSITORY https://github.com/apple/swift-asn1
+    GIT_TAG 1.1.0)
+  FetchContent_MakeAvailable(ASN1)
+endif()
+
+find_package(SwiftCrypto CONFIG)
+if(NOT SwiftCrypto_FOUND)
+  message("-- Vending swift-crypto")
+  FetchContent_Declare(Crypto
+    GIT_REPOSITORY https://github.com/apple/swift-crypto
+    GIT_TAG 3.0.0)
+  FetchContent_MakeAvailable(Crypto)
+endif()
+
+set(BUILD_TESTING ${_CERT_SAVED_BUILD_TESTING})
+set(BUILD_EXAMPLES ${_CERT_SAVED_BUILD_EXAMPLES})
 
 add_subdirectory(Sources)
 add_subdirectory(cmake/modules)