build: allow using a build tree of dispatch, Foundation (#67)

When building swift-crypto from source for a toolchain build, it is
convenient to build it without building up a toolchain image.  This
additional knob allows for building swift-crypto with the build
artifacts of previous builds, allowing building up a toolchain image
while chaining multiple builds together.

Co-authored-by: tomer doron <[email protected]>

Author: compnerd

CMakeLists.txt

@@ -37,5 +37,10 @@ if(BUILD_SHARED_LIBS)
   set(CMAKE_POSITION_INDEPENDENT_CODE YES)
 endif()
 
+if(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  find_package(dispatch CONFIG)
+  find_package(Foundation CONFIG)
+endif()
+
 add_subdirectory(Sources)
 add_subdirectory(cmake/modules)

Sources/Crypto/CMakeLists.txt

@@ -80,6 +80,8 @@ target_include_directories(Crypto PRIVATE
   $<TARGET_PROPERTY:CCryptoBoringSSL,INCLUDE_DIRECTORIES>
   $<TARGET_PROPERTY:CCryptoBoringSSLShims,INCLUDE_DIRECTORIES>)
 target_link_libraries(Crypto PUBLIC
+  $<$<NOT:$<PLATFORM_ID:Darwin>>:dispatch>
+  $<$<NOT:$<PLATFORM_ID:Darwin>>:Foundation>
   CCryptoBoringSSL
   CCryptoBoringSSLShims)
 set_target_properties(Crypto PROPERTIES