CMake Error at cmake/OpenCVCompilerOptions.cmake:21 (else):

CMake Error at cmake/OpenCVCompilerOptions.cmake:21 (else):
  A duplicate ELSE command was found inside an IF block.
Call Stack (most recent call first):
  CMakeLists.txt:495 (include)

这种问题很简单了，直接打开cmake/OpenCVCompilerOptions.cmake:21 ，找到21行，他就是多了一行多余的endif，直接删除掉就好了