How to fix 'Unknown CMake command "check_symbol_exists"'

If you have CMake code using check_symbol_exists(...) like

CMakeLists.txt

list(APPEND CMAKE_REQUIRED_LIBRARIES m)
check_symbol_exists(atan2 math.h HAVE_ATAN2)

list(APPEND CMAKE_REQUIRED_LIBRARIES m)
check_symbol_exists(atan2 math.h HAVE_ATAN2)

but you get an error message like

error_message.txt

CMake Error at CMakeLists.txt:8 (check_symbol_exists):
  Unknown CMake command "check_symbol_exists".

-- Configuring incomplete, errors occurred!

CMake Error at CMakeLists.txt:8 (check_symbol_exists):
  Unknown CMake command "check_symbol_exists".

-- Configuring incomplete, errors occurred!

you need to add

CMakeLists_fixed.txt

include(CheckSymbolExists)

include(CheckSymbolExists)

near the top of your CMakeLists.txt (before your first call to check_symbol_exists()).

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