* runtests.sh: avoid redundant `pkg-config` checks and simplified the check
* runtests.sh: write a message when something is completely missing
* runtests.sh: derive paths from script location
* runtests.sh: improved readability by using individual functions for each file
* runtests.sh: added helper function `get_pkg_config_cflags()` to clean up cflag lookup from `pkg-config`
* runtests.sh: simplified `xmlstarlet` calls