Miscellaneous build fixes

A couple of build fixes I implemented in a private fork last year, rebased on top of current github pycurl/pycurl master:

*    setup.py: detect ssl variant using ctypes/curl_version
*    pycurl.c: fix curl_easy_getinfo(CURLINFO_PRIVATE) argument type
*    pycurl: make internal functions static
*    pycurl: make doc and version strings const and read-only