Supports more two SSL libraries for NSS and GnuTLS.

This change supports more two SSL library which are NSS and GnuTLS.

The libcurl library which is called by s3fs is implemented in a libraries of OpenSSL or NSS, GnuTLS, etc.
Then s3fs should call same library as implemented in libcurl.

And maybe this change solves the problem of the OpenSSL license for building a package and distributing it.

Finally this change adds a new option about selecting libraries to configuration. Specifically, I will add a postscript in Installation Notes wiki page.