Cleanup CMake librabbitmq target_link_libraries()

Now have one variable to adjust when adding new libraries instead of
adding it in 2 different places. Also make sure static rabbitmq-c links
against the SSL libraries
CMake: make sure amqp_*_socket.h gets installed
autotools build needs pkg-config in README.md

The autotools build needs pkg-config. Add this to the README.md file.
Update amqp.h

Bug fixes, building with cmake & mingw on win32
FIX: memory leak in amqp_new_connection

amqp_new_connection will fail to free the amqp_connection_state_t object
that is allocated when amqp_tune_connection fails with ERROR_NO_MEMORY.
This fixes that
FIX: missing va_end in amqp_login* functions

Be sure to va_end any va_start'ed va_lists in amqp_login and
amqp_login_with_properties