add initial CMake and Visual Studio build support

This moves the compatibility include files from include to
include/compat so we can use the awful MS C compiler
<../include/> trick to emulate the GNU #include_next extension.

This also removes a few old compat files we do not need anymore.

patches/pkcs7.h.patch

@@ -1,15 +1,17 @@
 --- include/openssl/pkcs7.h.orig	2015-07-06 13:26:27.369203527 -0700
 +++ include/openssl/pkcs7.h	2015-07-06 13:27:37.637051967 -0700
-@@ -69,6 +69,16 @@
+@@ -69,6 +69,18 @@
  extern "C" {
  #endif
 
 +#if defined(_WIN32) && defined(__WINCRYPT_H__)
++#ifndef LIBRESSL_INTERNAL
 +#ifdef _MSC_VER
 +#pragma message("Warning, overriding WinCrypt defines")
 +#else
 +#warning overriding WinCrypt defines
 +#endif
++#endif
 +#undef PKCS7_ISSUER_AND_SERIAL
 +#undef PKCS7_SIGNER_INFO
 +#endif