Updated included headers to support compilation on Windows (using Visual C++)

- most of posix related functions and constants in unistd.h can be
  found in io.h in Visual C++
- introduced src/compat/msvc.h to adjust for compiler differences (and
  avoid updating code with #ifdef blocks for Windows support)
- removed some included headers that are not needed (both on Unix and
  Windows builds)

src/collection/backend/in_memory-per_process.h

@@ -12,6 +12,7 @@
  * directly using the email address security@modsecurity.org.
  *
  */
+#include <pthread.h>
 
 
 #ifdef __cplusplus

src/collection/backend/lmdb.cc

@@ -18,7 +18,11 @@
 #include "src/collection/backend/collection_data.h"
 
 #include <sys/types.h>
+#ifndef WIN32
 #include <unistd.h>
+#else
+#include <io.h>
+#endif
 
 #include <string>
 #include <memory>