CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_EXPORT_NO_PACKAGE_REGISTRY
    CMAKE_INSTALL_DATAROOTDIR
    CMAKE_INSTALL_LIBEXECDIR
    CMAKE_INSTALL_OLDINCLUDEDIR
    CMAKE_INSTALL_SBINDIR


In file included from /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/654e8bb96434ddfc35fed859212211e996782641/source_subfolder/src/imap.cpp:27:0:
/home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/654e8bb96434ddfc35fed859212211e996782641/source_subfolder/include/mailio/imap.hpp:19:20: fatal error: optional: No such file or directory
 #include <optional>
                    ^
compilation terminated.
make[2]: *** [source_subfolder/CMakeFiles/mailio.dir/src/imap.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [source_subfolder/CMakeFiles/mailio.dir/all] Error 2
make: *** [all] Error 2
mailio/0.20.0: WARN: Build folder is dirty, removing it: /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/654e8bb96434ddfc35fed859212211e996782641
mailio/0.20.0: ERROR: Package '654e8bb96434ddfc35fed859212211e996782641' build failed
mailio/0.20.0: WARN: Build folder /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/654e8bb96434ddfc35fed859212211e996782641
ERROR: mailio/0.20.0: Error in build() method, line 68
	cmake.build()
	ConanException: Error 2 while executing cmake --build '/home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/654e8bb96434ddfc35fed859212211e996782641/build_subfolder' '--' '-j3'