Add SPDX license identifiers to source code. To help the automatic scanners to identify the license of the source code, add SPDX license identifiers as specified at https://spdx.github.io/spdx-spec/appendix-V-using-SPDX-short-identifiers-in-source-files/ In our case, the line is: SPDX-License-Identifier: ISC for every file.
Fix static library build. When Ahven is build as a static library, we need to add linker option '-lgnarl' during the linking. Otherwise, we get a lot of errors about missing functions when compiling the actual test runner. Also, define Library_Interface at the top level, so the static build also has it. This fixes the include dir population.