cmake: Backport to work with CMake 3.0 again

Changes in commit 7867aaa9a0 (cmake: link curl to the OpenSSL targets
instead of lib absolute paths, 2018-07-17) and commit f826b4ce98 (cmake:
bumped minimum version to 3.4, 2018-07-19) required CMake 3.4 to fix
issue #2746.  This broke support for users on older versions of CMake
even if they just want to build curl and do not care whether transitive
dependencies work.

Backport the logic to work with CMake 3.0 again by implementing the
fix only when the version of CMake is at least 3.4.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e6dbb73f1e73136008436f5f844ebc4dff86dfca..26d4980fcb7e168b127163bd17bd9036531e243f 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,7 +38,7 @@
 # To check:
 # (From Daniel Stenberg) The cmake build selected to run gcc with -fPIC on my box while the plain configure script did not.
 # (From Daniel Stenberg) The gcc command line use neither -g nor any -O options. As a developer, I also treasure our configure scripts's --enable-debug option that sets a long range of "picky" compiler options.
-cmake_minimum_required(VERSION 3.4 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/CMake;${CMAKE_MODULE_PATH}")
 include(Utilities)
 include(Macros)
@@ -352,7 +352,16 @@ if(CMAKE_USE_OPENSSL)
   set(USE_OPENSSL ON)
   set(HAVE_LIBCRYPTO ON)
   set(HAVE_LIBSSL ON)
-  list(APPEND CURL_LIBS OpenSSL::SSL OpenSSL::Crypto)
+
+  # Depend on OpenSSL via imported targets if supported by the running
+  # version of CMake.  This allows our dependents to get our dependencies
+  # transitively.
+  if(NOT CMAKE_VERSION VERSION_LESS 3.4)
+    list(APPEND CURL_LIBS OpenSSL::SSL OpenSSL::Crypto)
+  else()
+    list(APPEND CURL_LIBS ${OPENSSL_LIBRARIES})
+    include_directories(${OPENSSL_INCLUDE_DIR})
+  endif()
 
   set(CMAKE_REQUIRED_INCLUDES ${OPENSSL_INCLUDE_DIR})
   check_include_file("openssl/crypto.h" HAVE_OPENSSL_CRYPTO_H)