Incorrect assignment of `HAS_FEENABLEEXCEPT`

Inside cmake/corsikaDefines.cmake, we use the following mechanism to set the HAS_FEENABLEEXCEPT variable:

include (CheckIncludeFileCXX)
CHECK_INCLUDE_FILE_CXX ("fenv.h" HAS_FEENABLEEXCEPT)

This just checks for the existence of fenv.h. What should actually be done is that we check for the actual existence of _feenableexcept, which could look something like:

include (CheckCXXSymbolExists)
check_symbol_exists(_feenableexcept "fenv.h" HAS_FEENABLEXCEPT)

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