This was previously used, but was incorrectly removed in 83d54192e97b
while removing old threads remnants.  Instead of using it conditionally
when threads are not used, we now set in unconditionally, as even with
thread pools enabled we never call OpenSSL functions in threads.

This fixes resulting binary when using --with-openssl with OpenSSL 1.1.0+
and without -lpthread linked (notably on FreeBSD without PCRE).

OpenSSL now uses pthread_atfork(), and this requires -lpthread on Linux
to compile.  Introduced NGX_LIBPTHREAD to add it as appropriate, similar
to existing NGX_LIBDL.

The http_rewrite module cannot be selected when http is disabled.
Fixed the PCRE check condition to avoid irrelevant check failure.
This is a regression from 4d874b4d82ed.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

OpenSSL 1.1.0 now uses normal "nmake; nmake install" instead of using
custom "ms\do_ms.bat" script and "ms\nt.mak" makefile.  And Configure
now requires --prefix to be absolute, and no longer derives --openssldir
from prefix (so it's specified explicitly).  Generated libraries are now
called "libcrypto.lib" and "libssl.lib" instead of "libeay32.lib"
and "ssleay32.lib".  Appropriate tests added to support both old and new
variants.

Additionally, openssl/lhash.h now triggers warning C4090 ('function' :
different 'const' qualifiers), so the warning was disabled.

In Perl 5.8.6 the default was switched to use putenv() when used as
embedded library unless "PL_use_safe_putenv = 0" is explicitly used
in the code.  Therefore, for modern versions of Perl it is no longer
necessary to restore previous environment when calling perl_destruct().

In collaboration with Ivan Poluyanov.

IPv6 now compiled-in automatically if support is found.  If there is a need
to disable it for some reason, --with-cc-opt="-DNGX_HAVE_INET6=0" can be used
for this.

Previously flags passed by --with-ld-opt were not used when building perl
module, which meant hardening flags provided by package build systems were not
applied.