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

include/libssh2.h

@@ -87,7 +87,12 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
-#ifdef _WIN32
+
+#if defined(_WIN32) || defined(WIN32)
+#define _LIBSSH2_WIN32
+#endif
+
+#ifdef _LIBSSH2_WIN32
 # include <basetsd.h>
 # include <winsock2.h>
 #endif
@@ -99,7 +104,7 @@ extern "C" {
 
 /* Allow alternate API prefix from CFLAGS or calling app */
 #ifndef LIBSSH2_API
-# ifdef WIN32
+# ifdef _LIBSSH2_WIN32
 #  if defined(LIBSSH2_EXPORTS) || defined(DLL_EXPORT) || \
       defined(_WINDLL) || defined(libssh2_shared_EXPORTS)
 #   ifdef LIBSSH2_LIBRARY
@@ -110,9 +115,9 @@ extern "C" {
 #  else
 #   define LIBSSH2_API
 #  endif
-# else /* !WIN32 */
+# else /* !_LIBSSH2_WIN32 */
 #  define LIBSSH2_API
-# endif /* WIN32 */
+# endif /* _LIBSSH2_WIN32 */
 #endif /* LIBSSH2_API */
 
 #ifdef HAVE_SYS_UIO_H
@@ -138,13 +143,13 @@ typedef unsigned long long libssh2_uint64_t;
 typedef long long libssh2_int64_t;
 #endif
 
-#ifdef WIN32
+#ifdef _LIBSSH2_WIN32
 typedef SOCKET libssh2_socket_t;
 #define LIBSSH2_INVALID_SOCKET INVALID_SOCKET
-#else /* !WIN32 */
+#else /* !_LIBSSH2_WIN32 */
 typedef int libssh2_socket_t;
 #define LIBSSH2_INVALID_SOCKET -1
-#endif /* WIN32 */
+#endif /* _LIBSSH2_WIN32 */
 
 /*
  * Determine whether there is small or large file support on windows.
@@ -170,7 +175,7 @@ typedef int libssh2_socket_t;
 #  undef LIBSSH2_USE_WIN32_LARGE_FILES
 #endif
 
-#if defined(_WIN32) && !defined(LIBSSH2_USE_WIN32_LARGE_FILES) && \
+#if defined(_LIBSSH2_WIN32) && !defined(LIBSSH2_USE_WIN32_LARGE_FILES) && \
     !defined(LIBSSH2_USE_WIN32_SMALL_FILES)
 #  define LIBSSH2_USE_WIN32_SMALL_FILES
 #endif