provide SPDX identifiers

- All files have prominent copyright and SPDX identifier
- If not embedded in the file, in the .reuse/dep5 file
- All used licenses are in LICENSES/ (not shipped in tarballs)
- A new REUSE CI job verify that all files are OK

Assisted-by: Viktor Szakats

Closes #1084

cmake/CheckFunctionExistsMayNeedLibrary.cmake

@@ -32,6 +32,8 @@
 # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 # USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 # OF SUCH DAMAGE.
+#
+# SPDX-License-Identifier: BSD-3-Clause
 
 
 # - check_function_exists_maybe_need_library(<function> <var> [lib1 ... libn])