Check for basename() is now done the same as other function checks

renamed generated config.h to curl_config.h in order to avoid clashes when libcurl is used with other projects which also have a config.h.

Refactor how libraries are checked for connect() function,
and check for connect() as it is done for other functions.

socket() and closesocket() prototypes might be in socket.h

check for socket() and closesocket() as it is done for other functions

HAVE_IOCTL_SIOCGIFADDR since it's needed to use SIOCGIFADDR and Watcom C
doesn't currently define it.

used, and define HAVE_IOCTL_SIOCGIFADDR if appropriate.

checks for inet_ntoa_r() inet_ntop() and inet_pton()

non-blocking mode, and decouple function detection from function capability.

 and CHECK_FUNC_GETADDRINFO_UNFREEABLE_AI_CANONNAME
internal to CHECK_FUNC_GETADDRINFO

function returns an addrinfo with an unfreeable ai_canonname member ptr.

function returns an addrinfo with an unfreeable ai_addr member ptr.

configure is unable of finding out if it has glibc-style or POSIX-style,
trying to ensure that this condition does not go unnoticed.

inet_pton function is available.

and when not crosscompiling verifies if it is IPv6 capable.

HAVE_INET_NTOP will only be defined when an IPv6 capable working
inet_ntop function is available.

check for winsock build target configurations