curl: Set build options the way we need for CMake

Set options added by the update to curl 8.12.1.

In particular, now that upstream curl's CMake code depends on
OpenSSL via imported targets from FindOpenSSL, we can let it
handle the transitive dependency on posix threads.

Source/Modules/CMakeBuildUtilities.cmake

@@ -159,6 +159,7 @@ else()
     set(CURL_CA_BUNDLE "" CACHE FILEPATH "Path to SSL CA Certificate Bundle")
     set(CURL_CA_PATH "" CACHE PATH "Path to SSL CA Certificate Directory")
     mark_as_advanced(CURL_CA_BUNDLE CURL_CA_PATH)
+    find_package(OpenSSL)
   endif()
   if(NOT CMAKE_USE_SYSTEM_NGHTTP2)
     # Tell curl's FindNGHTTP2 module to use our library.