core(lapack): fix build issues related to 'extern "C"'

cmake/OpenCVFindLAPACK.cmake

@@ -31,7 +31,7 @@ macro(ocv_lapack_check)
   else()
     # adding proxy opencv_lapack.h header
     set(CBLAS_H_PROXY_PATH ${CMAKE_BINARY_DIR}/opencv_lapack.h)
-    if(APPLE)
+    if((APPLE OR OPENCV_SKIP_LAPACK_EXTERN_C) AND NOT OPENCV_FORCE_LAPACK_EXTERN_C)
         set(_lapack_include_str_extern_C "")
         set(_lapack_include_str_extern_C_end "")
     else()

cmake/checks/lapack_check.cpp

+#include <complex.h>
 #include "opencv_lapack.h"
 
 static char* check_fn1 = (char*)sgesv_;

modules/core/src/hal_internal.cpp

@@ -46,6 +46,7 @@
 
 #ifdef HAVE_LAPACK
 
+#include <complex.h>
 #include "opencv_lapack.h"
 
 #include <cmath>