[INFRA] Proper project

[eseiler]

This PR:

• Makes seqan-std a CMake project
• seqan-std can now be added via CPM

In #9, I also added CI. I did have to make a separate PR for that to make the actions run on this PR.
It is a simple compile/header test:

• Including a header twice does not cause errors (include guard check)
• A header does not include itself (Compile a file with the header content and remove the include guard. If it includes itself/the original header, there will be multiple-definition-errors.)

Follow-up

I think I'll also change the test's main.cpp to include all headers in a follow-up PR. This way, we would catch if some headers have the same include guards.
I'll also try to add a test that checks whether the FILE_SET HEADERS does indeed contain all headers.

Comments

Why

Instead of adding an include/seqan-std directory, I opted for:

get_filename_component (SEQAN_STD_DIR_NAME "${CMAKE_CURRENT_SOURCE_DIR}" NAME)
if (NOT SEQAN_STD_DIR_NAME STREQUAL "seqan-std")
    message (FATAL_ERROR "The directory name must be 'seqan-std'.")
endif ()

file (REAL_PATH "${CMAKE_CURRENT_SOURCE_DIR}/.." SEQAN_STD_INCLUDE_DIR)

That is, the parent directory is the include directory. It allows for, e.g.

#include <seqan-std/chunk_view.hpp>

I haven't found any sources on whether this is considered bad style or not.

Alternative 1

I also tried to use an include/seqan-std directory. For CPM-usage that works just as fine.
The main disadvantage is that it's not trivial to use seqan-std as git subtree in seqan3 and hibf.

• Either:
  The includes would need to be

  #include <seqan3/contrib/seqan-std/include/seqan-std/chunk_view.hpp>

• Or:
  We would need to add seqan3/contrib/seqan-std to the include path.

I don't like either option much.
By keeping the headers in the top-level directory, we don't need to change anything about the git subtree usage.

Alternative 2

An alternative would be to just include seqan-std via CPM.
But because we do not install seqan-std, we would have to ship it in our source packages or when installing seqan3/hibf.

Here is what this would look like: eseiler/seqan3@713ca0e
And here is a patch for posterity: seqan3_use_seqan-std_via_CPM.patch
While it works, it's far too much complexity.