Automatically use OpenSSL by default on Linux and FreeBSD if available

Since https is almost ubiquitous nowadays we should support it by
default whenever possible.  When building our own curl, we already
automatically enable SSL/TLS support on Windows and OS X by using the
OS-native APIs.  On UNIX platforms we need to use OpenSSL but have not
done so by default before, leading to possible user confusion when https
transfers fail later.  Fix this by searching for OpenSSL quietly and
enabling use of it automatically if it is found.

Do this only on Linux and FreeBSD for now because on other UNIX
platforms (e.g. AIX, HP-UX, SunOS) it seems too easy to find an
OpenSSL that is not compatible with the target compiler.

CMakeLists.txt

@@ -327,7 +327,15 @@ macro (CMAKE_BUILD_UTILITIES)
     if(CMAKE_TESTS_CDASH_SERVER)
       set(CMAKE_CURL_TEST_URL "${CMAKE_TESTS_CDASH_SERVER}/user.php")
     endif()
-    option(CMAKE_USE_OPENSSL "Use OpenSSL." OFF)
+    set(_CMAKE_USE_OPENSSL_DEFAULT OFF)
+    if(NOT DEFINED CMAKE_USE_OPENSSL AND NOT WIN32 AND NOT APPLE
+        AND CMAKE_SYSTEM_NAME MATCHES "(Linux|FreeBSD)")
+      find_package(OpenSSL QUIET)
+      if(OPENSSL_FOUND)
+        set(_CMAKE_USE_OPENSSL_DEFAULT ON)
+      endif()
+    endif()
+    option(CMAKE_USE_OPENSSL "Use OpenSSL." ${_CMAKE_USE_OPENSSL_DEFAULT})
     mark_as_advanced(CMAKE_USE_OPENSSL)
     if(CMAKE_USE_OPENSSL)
       set(CURL_CA_BUNDLE "" CACHE FILEPATH "Path to SSL CA Certificate Bundle")

Help/release/dev/enable-ssl-automatically.rst

@@ -0,0 +1,8 @@
+enable-ssl-automatically
+------------------------
+
+* On Linux and FreeBSD platforms, when building CMake itself from source and
+  not using a system-provided libcurl, OpenSSL is now used by default if it is
+  found on the system.  This enables SSL/TLS support for commands supporting
+  network communication via ``https``, such as :command:`file(DOWNLOAD)`,
+  :command:`file(UPLOAD)`, and :command:`ctest_submit`.