* runtests.sh: added strict mode - enabled via `STRICT=1`
* CI-unixish.yml: added strict cfg check
* runtests.sh: disabled syntax check for kde.cpp since `kde-4config` is no longer commonly available in recent distros
* runtests.sh: disabled syntax check for opencv2.cpp since `opencv.pc` is not commonly available in distros
* runtests.sh: added missing trailing slash to `CFG` - fixes define checks