Fix build on appveyor ci with mingw

C:/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/bin/c++.exe  (ver 8.1.0)
pthreads_cross.h:37:26: error: conflicting declaration 'typedef CRITICAL_SECTION pthread_mutex_t'
pthreads_cross.c:110:5: warning: implicit declaration of function 'InitializeConditionVariable'

3rdparty/apriltag/common/pthreads_cross.c

@@ -22,7 +22,8 @@ SOFTWARE.
 
 #include "common/pthreads_cross.h"
 
-#ifdef _WIN32
+//#ifdef _WIN32
+#if defined(_WIN32) && !defined(__MINGW32__)
 
 typedef struct {
     SRWLOCK lock;
@@ -246,11 +247,11 @@ unsigned int pcthread_get_num_procs()
     return sysinfo.dwNumberOfProcessors;
 }
 
-#else
+//#else
 
-#include <unistd.h>
-unsigned int pcthread_get_num_procs()
-{
-    return (unsigned int)sysconf(_SC_NPROCESSORS_ONLN);
-}
+//#include <unistd.h>
+//unsigned int pcthread_get_num_procs()
+//{
+//    return (unsigned int)sysconf(_SC_NPROCESSORS_ONLN);
+//}
 #endif

3rdparty/apriltag/common/pthreads_cross.h

@@ -23,7 +23,8 @@ SOFTWARE.
 #ifndef __CPTHREAD_H__
 #define __CPTHREAD_H__
 
-#ifdef _WIN32
+//#ifdef _WIN32
+#if defined(_WIN32) && !defined(__MINGW32__)
 #include <stdbool.h>
 #include <windows.h>
 #else
@@ -32,7 +33,8 @@ SOFTWARE.
 #endif
 #include <time.h>
 
-#ifdef _WIN32
+//#ifdef _WIN32
+#if defined(_WIN32) && !defined(__MINGW32__)
 
 typedef CRITICAL_SECTION pthread_mutex_t;
 typedef void pthread_mutexattr_t;

3rdparty/apriltag/common/time_util.h

@@ -36,7 +36,8 @@ either expressed or implied, of the Regents of The University of Michigan.
 typedef long long suseconds_t;
 #endif
 
-#ifdef _MSC_VER
+//#ifdef _MSC_VER
+#if defined(_MSC_VER) && defined(__MINGW32__)
 
 inline int gettimeofday(struct timeval* tp, void* tzp)
 {