We experienced weired linker errors when using host compiler version for
compilation that were not supported by the nvcc used to compile the
cuda code:
```
[ 15%] Linking CXX executable bin/test_timer
/usr/bin/ld: /home/mblatt/src/dune/opm-2.6/opm-common/opm-seq/lib/libopmcommon.a(Parser.cpp.o): in function `Opm::(anonymous namespace)::file& std::vector<Opm::(anonymous namespace)::file, std::allocator<Opm::(anonymous namespace)::file> >::emplace_back<std::filesystem::__cxx11::path&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>(std::filesystem::__cxx11::path&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)':
Parser.cpp:(.text+0x1096): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
/usr/bin/ld: Parser.cpp:(.text+0x10ad): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
/usr/bin/ld: /home/mblatt/src/dune/opm-2.6/opm-common/opm-seq/lib/libopmcommon.a(Parser.cpp.o): in function `Opm::(anonymous namespace)::ParserState::loadFile(std::filesystem::__cxx11::path const&)':
Parser.cpp:(.text+0x23a1): undefined reference to `std::filesystem::canonical(std::filesystem::__cxx11::path const&)'
/usr/bin/ld: Parser.cpp:(.text+0x24e0): undefined reference to
`std::filesystem::__cxx11::path::_M_split_cmpts()'
```
The reason turned out to be that the library path was build up by
paths of the old (g++-7) compiler used by nvcc and the actual (newer) compiler
g++-8. This completely messed up the linker paths for CMake.
To detect this situation already when running cmake we have resorted
to first setting the CMAKE_CUDA_FLAGS to force cmake to make nvcc use
the host compiler and to activate CUDA (if available) before calling
`find_package(CUDA)`. If the host compiler is not supported CMake will
error out during `enable(CUDA)`
Note that we still use (deprecated) FindCUDA later to determine the
libraries to link to.
The users has the option to either deactivate CUDA by setting
`-DCMAKE_DISABLE_FIND_PACKAGE_CUDA=ON` or to use a compiler supported
by nvcc (setting `-DCMAKE_CXX_COMPILER=compiler`).
Additionally we do not try to activate CUDA the CMake version is <
3.8. Please note that previously CMake would have errored out here
anyway since we used the unsupported `enable_language(CUDA)` even in
this case.
Closes#2363.
strictly speaking the gcc diagnostics are wrong, the typedef is actually
used once templates are invoked on the structs.
simply use the typedef in the structs itself to avoid the warning
like is done for other variants.
