Request: post installation runnable tests

Hello,



I'm updating the spdlog package for Debian and I'm no longer easily able to run the tests against the installed headers due to the removal of the standalone `tests/Makefile`.



Here is the current setup

```

# this is after copying the contents of the tests directory from the libspdlog-dev package to the # temporary working directory

cmake -DSPDLOG_BUILD_TESTS=ON -DSPDLOG_BUILD_TESTS_HO=ON .

make CFLAGS=-DSPDLOG_FMT_EXTERNAL

make test

```



And the current result



```

-- The CXX compiler identification is GNU 9.2.1

-- Check for working CXX compiler: /usr/bin/c++

-- Check for working CXX compiler: /usr/bin/c++ -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29") 

-- Checking for module 'libsystemd'

--   Found libsystemd, version 242

CMake Warning (dev) in CMakeLists.txt:

  No cmake_minimum_required command is present.  A line of code such as



    cmake_minimum_required(VERSION 3.13)



  should be added at the top of the file.  The version specified may be lower

  if you wish to support older CMake versions for this project.  For more

  information run "cmake --help-policy CMP0000".

This warning is for project developers.  Use -Wno-dev to suppress it.



-- Configuring done

CMake Warning (dev) at CMakeLists.txt:58 (add_executable):

  Policy CMP0028 is not set: Double colon in target name means ALIAS or

  IMPORTED target.  Run "cmake --help-policy CMP0028" for policy details.

  Use the cmake_policy command to set the policy and suppress this warning.



  Target "spdlog-utests-ho" links to target "spdlog::spdlog_header_only" but

  the target was not found.  Perhaps a find_package() call is missing for an

  IMPORTED target, or an ALIAS target is missing?

This warning is for project developers.  Use -Wno-dev to suppress it.



-- Generating done

-- Build files have been written to: /tmp/autopkgtest.u6TQBq/autopkgtest_tmp/tests

Scanning dependencies of target spdlog-utests

[  2%] Building CXX object CMakeFiles/spdlog-utests.dir/test_file_helper.o

In file included from /usr/include/spdlog/common.h:38,

                 from /usr/include/spdlog/spdlog.h:12,

                 from /tmp/autopkgtest.u6TQBq/autopkgtest_tmp/tests/includes.h:17,

                 from /tmp/autopkgtest.u6TQBq/autopkgtest_tmp/tests/test_file_helper.cpp:4:

/usr/include/spdlog/fmt/fmt.h:22:10: fatal error: bundled/core.h: No such file or directory

   22 | #include "bundled/core.h"

      |          ^~~~~~~~~~~~~~~~

compilation terminated.

```



```

[
  {
    "category": "underspecification",
    "rule": "signature.missing_system_requirement",
    "source": "failure_signature",
    "excerpt": "/usr/include/spdlog/fmt/fmt.h:22:10: fatal error: bundled/core.h: No such file or directory"
  }
]