build: workaround issues with C compilation mode

- cvdef.h + cvRound (double only)
- highgui_c.h

modules/core/include/opencv2/core/cvdef.h

@@ -480,7 +480,7 @@ Cv64suf;
 // Integer types portatibility
 #ifdef OPENCV_STDINT_HEADER
 #include OPENCV_STDINT_HEADER
-#else
+#elif defined(__cplusplus)
 #if defined(_MSC_VER) && _MSC_VER < 1600 /* MSVS 2010 */
 namespace cv {
 typedef signed char int8_t;
@@ -517,9 +517,15 @@ typedef ::int64_t int64_t;
 typedef ::uint64_t uint64_t;
 }
 #endif
+#else // pure C
+#include <stdint.h>
 #endif
 
 
 //! @}
 
+#ifndef __cplusplus
+#include "opencv2/core/fast_math.hpp" // define cvRound(double)
+#endif
+
 #endif // OPENCV_CORE_CVDEF_H