several[1][2] build systems use a variable called "srcdir"
to locate either the root project directory, or some subdirectory

we should not blindly export a variable under the same name,
as it leads to broken builds - either loudly, when tests fail,
or quietly when a ""missing"" file simply gets ignored

[1]: https://github.com/PCRE2Project/pcre2/blob/pcre2-10.45/RunTest#L141
[2]: https://github.com/json-c/json-c/blob/json-c-0.18/tests/test-defs.sh#12