install-sbbs.mk doesn't complete successfully when SBSBDIR or REPODIR are provided with relative paths

I think some GNU make special assignment with $(shell realpath ...) would help here, but didn't figure it out yet and didn't want to fuggetabadit.

e.g.

$ make -f ~/sbbs/install/install-sbbs.mk SBBSDIR=../test2 run
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
make: ../test2/repo: No such file or directory
cp -r /ctrl ../test2
cp: cannot stat '/ctrl': No such file or directory
make: *** [/home/rswindell/sbbs/install/install-sbbs.mk:264: ../test2/ctrl] Error 1

and when REPODIR is relative, you end up with something like this:

make -C /home/rswindell/test2/exec MAKEFLAGS= os=linux RELEASE=1 BAJAPATH=../repo2/src/sbbs3/gcc.linux.x64.exe.release/baja
make[1]: Entering directory '/home/rswindell/test2/exec'
make[1]: *** No rule to make target 'ftp.bin', needed by 'all'.  Stop.