cmake: enforce USE_SSH validity

Validate the USE_SSH option fits into our valid options; don't
assume a default.

Author: ethomson

CMakeLists.txt

@@ -30,7 +30,7 @@ option(USE_THREADS             "Use threads for parallel processing when possibl
 option(USE_NSEC                "Support nanosecond precision file mtimes and ctimes"    ON)
 
 # Backend selection
-option(USE_SSH                 "Enable SSH support. Can be set to a specific backend" OFF)
+   set(USE_SSH                 "" CACHE STRING "Enables SSH support. One of libssh2, exec, or OFF. (Defaults to OFF.)")
 option(USE_HTTPS               "Enable HTTPS support. Can be set to a specific backend" ON)
 option(USE_SHA1                "Enable SHA1. Can be set to CollisionDetection(ON)/HTTPS" ON)
 option(USE_SHA256              "Enable SHA256. Can be set to HTTPS/Builtin" ON)

cmake/SelectSSH.cmake

@@ -39,6 +39,8 @@ elseif(USE_SSH STREQUAL ON OR USE_SSH STREQUAL "libssh2")
 	set(GIT_SSH 1)
 	set(GIT_SSH_LIBSSH2 1)
 	add_feature_info(SSH ON "using libssh2")
-else()
+elseif(USE_SSH STREQUAL OFF OR USE_SSH STREQUAL "")
 	add_feature_info(SSH OFF "SSH transport support")
+else()
+	message(FATAL_ERROR "unknown SSH option: ${USE_HTTP_PARSER}")
 endif()