cmake,options: Allow to set global client config file

This allows the global client (ssh_session) configuration file path to
be set in configuration time by defining GLOBAL_CLIENT_CONFIG when
calling cmake.  If it is not defined, the default path is set as
"/etc/ssh/ssh_config".

usage example:

$ cmake -DGLOBAL_CLIENT_CONFIG=/etc/my/custom/path ..

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

CMakeLists.txt

@@ -243,6 +243,7 @@ message(STATUS "Benchmarks: ${WITH_BENCHMARKS}")
 message(STATUS "Symbol versioning: ${WITH_SYMBOL_VERSIONING}")
 message(STATUS "Allow ABI break: ${WITH_ABI_BREAK}")
 message(STATUS "Release is final: ${WITH_FINAL}")
+message(STATUS "Global client config: ${GLOBAL_CLIENT_CONFIG}")
 if (WITH_SERVER)
 message(STATUS "Global bind config: ${GLOBAL_BIND_CONFIG}")
 endif()