The PCRE2 library is now used by default if found, instead of the
original PCRE library.  If needed for some reason, this can be disabled
with the --without-pcre2 configure option.

To make it possible to specify paths to the library and include files
via --with-cc-opt / --with-ld-opt, the library is first tested without
any additional paths and options.  If this fails, the pcre2-config script
is used.

Similarly to the original PCRE library, it is now possible to build PCRE2
from sources with nginx configure, by using the --with-pcre= option.
It automatically detects if PCRE or PCRE2 sources are provided.

Note that compiling PCRE2 10.33 and later requires inttypes.h.  When
compiling on Windows with MSVC, inttypes.h is only available starting
with MSVC 2013.  In older versions some replacement needs to be provided
("echo '#include <stdint.h>' > pcre2-10.xx/src/inttypes.h" is good enough
for MSVC 2010).

The interface on nginx side remains unchanged.

Now we always set NGX_CC_NAME to "msvc", and additionally test compiler
version as reported by "cl" in auto/cc/msvc (the same version is also
available via the _MSC_VER define).  In particular, this approach allows
to properly check for C99 variadic macros support, which previously was
not used with MSVC versions not explicitly recognized.

Now unneeded wildcards in NGX_CC_NAME tests for msvc removed accordingly,
as well as unused wildcards for owc and icc.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.

Previously, an attempt to build pcre.lib on win32 before anything else
failed due to no pcre.h.

configuration phase.

instead of single old 4.4 version

instead, do chdir inside Makefile

options as --with-zlib=../zlib-1.2.3.  It seems there is no common way
to learn the current directory in Win32 make's: although nmake has MAKEDIR
variable, nevertheless Borland make's MAKEDIR is the directory where make
is installed, and OpenWatcom wmake has no MAKEDIR at all.