Finally get rid of CURL_CHECK_HEADERS_ONCE since it adds very little value and has portability issues.

Change some shell if...then...fi tests into case...esac tests which demand less resources.

Using backslashes and slashes in the strings of the sed 'y' command shall be avoided since its interpretation is not the same across platforms.

Now we use the sed 's' command with a bracket expression.

effect.

Prevent definition of HAVE_WINxxx_H symbols and avoid inclusion of Windows headers when compiled with Cygwin in POSIX emulation mode.

So avoid using x_was_used().

(Yang Tse forgot about djgpp)

HAVE_GETNAMEINFO, GETNAMEINFO_QUAL_ARG1, GETNAMEINFO_TYPE_ARG1, GETNAMEINFO_TYPE_ARG2, GETNAMEINFO_TYPE_ARG46, GETNAMEINFO_TYPE_ARG7

HAVE_RECV, RECV_TYPE_ARG1, RECV_TYPE_ARG2, RECV_TYPE_ARG3, RECV_TYPE_ARG4, RECV_TYPE_RETV

HAVE_SEND, SEND_TYPE_ARG1, SEND_TYPE_ARG2, SEND_TYPE_ARG3, SEND_TYPE_ARG4, SEND_TYPE_RETV

Proxy-Connection: header when using a proxy and not doing CONNECT.