support ruby 2.1.0-preview1.
(requested by Yasuo Honda)
support rubinius 2.x whose RUBY_VERSION is 2.1.0.
reduce the size of libOCI.a created for cygwin.
append the full path of the Oracle client library when OCIEnvCreate
is failed such as "OCI Library Initialization Error - /path/to/libclntsh.so.12.1".