Makefile: do not depend on curl-config

MinGW builds of cURL does not ship with curl-config unless built
with the autoconf based build system, which is not the practice
recommended by the documentation. MsysGit has had issues with
binaries of that sort, so it has switched away from autoconf-based
cURL-builds.

Unfortunately, broke pushing over WebDAV on Windows, because
http-push.c depends on cURL's multi-threaded API, which we could
not determine the presence of any more.

Since troublesome curl-versions are ancient, and not even present
in RedHat 5, let's just assume cURL is capable instead of doing a
non-robust check.

Instead, add a check for curl_multi_init to our configure-script,
for those on ancient system. They probably already need to do the
configure-dance anyway.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>