setup: Do not fail when pkg-config is missing

[bdrung]

$ ./setup.py --help
Traceback (most recent call last):
  File "./setup.py", line 106, in <module>
    systemd_unit_dir = subprocess.check_output(
  File "/usr/lib/python3.10/subprocess.py", line 421, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.10/subprocess.py", line 503, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.10/subprocess.py", line 971, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.10/subprocess.py", line 1863, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'pkg-config'

[codecov]

Codecov Report

Merging #229 (4f7e791) into main (d20d68b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #229   +/-   ##
=======================================
  Coverage   82.98%   82.98%           
=======================================
  Files          92       92           
  Lines       18748    18748           
=======================================
  Hits        15558    15558           
  Misses       3190     3190           

Files	Coverage Δ
setup.py	86.95% <100.00%> (ø)