In some cases, it was possible for the include directory of the system-wide
libcurl to be added to the include path before cmcurl's, which would result
in them being picked up and causing the build to fail if the curl versions
differ too much.
One way to trigger this is to have OpenSSL installed into a non-default
location together with libcurl (/usr/local, for example). If cmcurl is built
with CMAKE_USE_OPENSSL on, -I/usr/local/include would end up being added
before -I${PATH_TO_CMCURL}.
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