cmake: add DEBUG_PRE/DEBUG_POST commands handling

Useful for debug purposes:
    cmake -DDEBUG_POST="ocv_cmake_dump_vars(CUDA)" .
    cmake -DCMAKE_PRE="ocv_cmake_dump_vars(\"OPENCL|opencl\")" .

CMakeLists.txt

@@ -73,6 +73,8 @@ endif()
 
 include(cmake/OpenCVUtils.cmake)
 
+ocv_cmake_eval(DEBUG_PRE ONCE)
+
 ocv_clear_vars(OpenCVModules_TARGETS)
 
 # ----------------------------------------------------------------------------
@@ -1110,3 +1112,4 @@ include(cmake/OpenCVPackaging.cmake)
 
 # This should be the last command
 ocv_cmake_dump_vars("" TOFILE "CMakeVars.txt")
+ocv_cmake_eval(DEBUG_POST ONCE)

cmake/OpenCVUtils.cmake

@@ -42,6 +42,16 @@ function(ocv_cmake_dump_vars)
   endif()
 endfunction()
 
+function(ocv_cmake_eval var_name)
+  if(DEFINED ${var_name})
+    file(WRITE "${CMAKE_BINARY_DIR}/CMakeCommand-${var_name}.cmake" ${${var_name}})
+    include("${CMAKE_BINARY_DIR}/CMakeCommand-${var_name}.cmake")
+  endif()
+  if(";${ARGN};" MATCHES ";ONCE;")
+    unset(${var_name} CACHE)
+  endif()
+endfunction()
+
 # assert macro
 # Note: it doesn't support lists in arguments
 # Usage samples: