Merge pull request #19985 from fpetrogalli:disable_threads

* [build][option] Introduce `OPENCV_DISABLE_THREAD_SUPPORT` option.

The option forces the library to build without thread support.

* update handling of OPENCV_DISABLE_THREAD_SUPPORT

- reduce amount of #if conditions

* [to squash] cmake: apply mode vars in toolchains too

Co-authored-by: Alexander Alekhin <alexander.a.alekhin@gmail.com>

modules/core/src/parallel.cpp

@@ -72,7 +72,7 @@
     #endif
 #endif
 
-#if defined CV_CXX11
+#ifndef OPENCV_DISABLE_THREAD_SUPPORT
     #include <thread>
 #endif
 
@@ -884,6 +884,7 @@ T minNonZero(const T& val_1, const T& val_2)
     return (val_1 != 0) ? val_1 : val_2;
 }
 
+#ifndef OPENCV_DISABLE_THREAD_SUPPORT
 static
 int getNumberOfCPUs_()
 {
@@ -986,6 +987,13 @@ int getNumberOfCPUs()
     return nCPUs;  // cached value
 }
 
+#else  // OPENCV_DISABLE_THREAD_SUPPORT
+int getNumberOfCPUs()
+{
+    return 1;
+}
+#endif  // OPENCV_DISABLE_THREAD_SUPPORT
+
 const char* currentParallelFramework()
 {
     std::shared_ptr<ParallelForAPI>& api = getCurrentParallelForAPI();