Refactor how preprocessor symbol _THREAD_SAFE definition is done.

OpenSSL 0.9.7 or newer required for ENGINE_CTRL_GET_CMD_FROM_NAME definition

  --with-nss is set but not "yes".

  I think we can still improve that to check for pkg-config in that path etc,
  but at least this patch brings back the same functionality we had before.

  the client certificate. It also disable the key name test as some engines
  can select a private key/cert automatically (When there is only one key
  and/or certificate on the hardware device used by the engine)

- Constantine Sapuntzakis provided the fix that ensures that an SSL connection
  won't be reused unless protection level for peer and host verification match.

knowledge says the contrary or h_errno is not defined.

- Constantine Sapuntzakis reported that Darwin 6.0 a.k.a. MAC OS X 10.2
  and newer have a threadsafe getaddrinfo.

- Fix Dragonfly BSD triplet detection.

- In case the hard-coded knowledge says that getaddrinfo is threadsafe,
  an additional check is done to verify that h_errno is also defined.
  If h_errno isn't defined, we finally assume that it isn't threadsafe.
  Jamie Lokier provided the inspiration for this extra check.

Add some comments to better understand what the regex's pretend to achieve.

No need for a separate variable ndns.

The memory leak detection will detect code that fails to release a dns reference.

The DEBUGASSERT will detect code that releases too many references.

  a broken TLS server. However it does not happen if SSL version is selected
  manually. The approach was originally taken from PSM. Kaspar Brand helped me
  to complete the patch. Original bug reports:
  https://bugzilla.redhat.com/525496
  https://bugzilla.redhat.com/527771

  closed NSPR descriptor. The issue was hard to find, reported several times
  before and always closed unresolved. More info at the RH bug:
  https://bugzilla.redhat.com/534176

  and GNU GSS installed due to a missing mutual exclusion of header files in
  the Kerberos 5 code path. He also verified that my patch worked for him.

  (http://curl.haxx.se/bug/view.cgi?id=2891595) which identified how an entry
  in the DNS cache would linger too long if the request that added it was in
  use that long. He also provided the patch that now makes libcurl capable of
  still doing a request while the DNS hash entry may get timed out.

  used during the FTP connection phase (after the actual TCP connect), while
  it of course should be. I also made the speed check get called correctly so
  that really slow servers will trigger that properly too.

c-ares libraries in debug and release flavours.

Additionally each of the three sample programs is built against
each of the four possible c-ares libraries, generating all this
a total number of 12 executables and 4 libraries.

  in non-blocking mode.

curl.h, adjusting auto-makefiles include path, to enhance portability to
OS's without an orthogonal directory tree structure such as OS/400.

  wrong percentage for small files, most notable for <1000 bytes and could
  easily end up showing more than 100% at the end. It also didn't show any
  percentage, transfer size or estimated transfer times when transferring
  less than 100 bytes.