cmake: verify minimum CMake version in libssh2-config.cmake

This is the same minimum version required to build libssh2 itself (3.7).

Closes #1754

cmake/libssh2-config.cmake.in

@@ -4,6 +4,11 @@
 option(LIBSSH2_USE_PKGCONFIG "Enable pkg-config to detect @PROJECT_NAME@ dependencies. Default: @LIBSSH2_USE_PKGCONFIG@"
   "@LIBSSH2_USE_PKGCONFIG@")
 
+if(CMAKE_VERSION VERSION_LESS @CMAKE_MINIMUM_REQUIRED_VERSION@)
+  message(STATUS "@PROJECT_NAME@: @PROJECT_NAME@-specific Find modules require "
+    "CMake @CMAKE_MINIMUM_REQUIRED_VERSION@ or upper, found: ${CMAKE_VERSION}.")
+endif()
+
 include(CMakeFindDependencyMacro)
 
 set(_libssh2_cmake_module_path_save ${CMAKE_MODULE_PATH})

src/CMakeLists.txt

@@ -397,6 +397,7 @@ if(NOT LIBSSH2_DISABLE_INSTALL)
     DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 
   # Consumed variables:
+  #   CMAKE_MINIMUM_REQUIRED_VERSION
   #   CRYPTO_BACKEND
   #   LIB_NAME
   #   LIB_SELECTED