Trying to add parallel for loops (with pthread)

- OpenCV lib compiled with PTHREADS_PF=ON
- Copied and tested in esp32/examples/esp_opencv_tests/
- TODO: Problem while linking pthreads :
        undefined reference to `pthread_cond_signal`

modules/core/src/parallel_impl.cpp

@@ -7,7 +7,13 @@
 #include "parallel_impl.hpp"
 
 #ifdef HAVE_PTHREADS_PF
+#ifdef __cplusplus
+extern "C" {
+#endif
 #include <pthread.h>
+#ifdef __cplusplus
+}
+#endif
 
 #include <opencv2/core/utils/configuration.private.hpp>