autoconf: Checks for libraries

autoconf: Checks for libraries

./configure script checks now if the following libraries are present:
 * -lssl    for SHA1_Init (NO_OPENSSL)
 * -lcurl   for curl_easy_setopt (NO_CURL)
 * -lexpat  for XML_ParserCreate (NO_EXPAT)
It also checks if adding the following libraries are needed:
 * -lcrypto for SHA1_Init (NEEDS_SSL_WITH_CRYPTO)
 * -liconv  for iconv (NEEDS_LIBICONV)
 * -lsocket for socket (NEEDS_SOCKET)

Policy: we check also if NEEDS_LIBRARY libraries are present, even if
there is no NO_LIBRARY variable.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>