build: hand-crafted config rework & header tidy-up

- introduce the concept of a project level setup header
  `src/libssh2_setup.h`, that is used by `src`, `example` and `tests`
  alike. Move there all common platform/compiler configuration from
  `src/libssh2_priv.h`, individual sources and `CMakeFiles.txt` files.
  Also move there our hand-crafted (= not auto-generated by CMake or
  autotools) configuration `win32/libssh2-config.h`.

- `win32` directory is empty now, delete it.

- `Makefile.mk`: adapt to the above. Build-directory is the target
  triplet, or any custom name set via `BLD_DIR`.

- sync header path order between build systems:
  build/src -> source/src -> source/include

- delete redundant references to `windows.h`, `winsock2.h`,
  `ws2tcpip.h`.

- delete unnecessary #includes, update order (`libssh2_setup.h` first,
  `winsock2.h` first), simplify where possible.

  This makes the code warning-free without `WIN32_LEAN_AND_MEAN`.
  At the same time this patch applies this macro globally, to avoid
  header bloat.

- example: add missing *nix header guards.

- example: fix misindented `HAVE_UNISTD_H` `#ifdef`s.

- set `WIN32` with all build-tools.

- set `HAVE_SYS_PARAM_H` in the hand-crafted config for MinGW.
  To match auto-detection.

- move a source-specific macro to `misc.c` from `libssh2_priv.h`.

See the PR's individual commits for step-by-step updates.

Closes #932

src/libssh2_priv.h

@@ -39,50 +39,12 @@
  * OF SUCH DAMAGE.
  */
 
-/* Disable warnings: C4127: conditional expression is constant */
-#if defined(_MSC_VER) && _MSC_VER < 1900
-#pragma warning(disable:4127)
-#endif
-
-/* Define mingw-w64 version macros, eg __MINGW{32,64}_{MINOR,MAJOR}_VERSION */
-#ifdef __MINGW32__
-#include <_mingw.h>
-#endif
+/* Header used by 'src' */
 
 #define LIBSSH2_LIBRARY
-#include "libssh2_config.h"
 
-/* Number of bits in a file offset, on hosts where this is settable. */
-#if defined(__MINGW32__) && defined(__MINGW64_VERSION_MAJOR)
-# ifndef _FILE_OFFSET_BITS
-# define _FILE_OFFSET_BITS 64
-# endif
-#endif
-
-#ifdef WIN32
-#ifndef WIN32_LEAN_AND_MEAN
-#define WIN32_LEAN_AND_MEAN
-#endif
-#include <windows.h>
-#undef WIN32_LEAN_AND_MEAN
-
-/* Detect Windows App environment which has a restricted access
-   to the Win32 APIs. */
-# if (defined(_WIN32_WINNT) && (_WIN32_WINNT >= 0x0602)) || \
-  defined(WINAPI_FAMILY)
-#  include <winapifamily.h>
-#  if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) &&  \
-     !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
-#    define LIBSSH2_WINDOWS_APP
-#  endif
-# endif
-
-#include <winsock2.h>
-#include <ws2tcpip.h>
-/* Force parameter type. */
-#define recv(s, b, l, f)  recv((s), (b), (int)(l), (f))
-#define send(s, b, l, f)  send((s), (b), (int)(l), (f))
-#endif
+/* platform/compiler-specific setup */
+#include "libssh2_setup.h"
 
 #include <stdio.h>
 #include <time.h>
@@ -128,6 +90,19 @@
 #include "libssh2_sftp.h"
 #include "misc.h"
 
+#ifdef WIN32
+/* Detect Windows App environment which has a restricted access
+   to the Win32 APIs. */
+# if (defined(_WIN32_WINNT) && (_WIN32_WINNT >= 0x0602)) || \
+  defined(WINAPI_FAMILY)
+#  include <winapifamily.h>
+#  if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) &&  \
+     !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+#    define LIBSSH2_WINDOWS_APP
+#  endif
+# endif
+#endif
+
 #ifndef FALSE
 #define FALSE 0
 #endif