If you do the build in advance, outside of the snap environment, the environment variables used by snapcraft will not be injected into your snap…
I’d start with translating the build instructions into actual snapcraft commands using the make plugin so library search paths, linker and include paths are correct …
then it seems like PYLON_ROOT needs to point to the place where your install lives … so add something to your apps: entry for this:
apps:
hellobasler:
environment:
PYLON_ROOT: $SNAP/path/to/pylon/install
command: bin/Grab
watching jounralctl -f or dmesg -w for denials like james suggested is also always helpful when developing snaps.