Updates allowing to improve building on linux.

[t-w]

Main changes:

1. allow to build only tools that do not require opencbm (nibconv, nibrepair, nibscan)
   • these tools can be used by more people than the tools for transferring data from a C= drive (not many people have the hardware...)
   • this is just as a quick change, to make it possible (otherwise heavy changes in build configurations needed...)
   • made these tools build first (so that the build fails after building these)
   • minimized dependencies (includes and link) so that opencbm is not required (where not needed/used)
2. path to opencbm can be set in LINUX/Makefile
3. added main makefile where the build env. can be selected (then just make / make clean etc.)
4. couple of other minor fixes/improvements.

Other notes:

1. I am not sure where to open a PR, if this is not the right branch let me know where (if you want to merge it).

2. Note that these changes are tested (briefly) only on Debian Linux. The changes are not big so should not make much trouble, but I cannot guarantee that eg. on DJGPP something unexpected won't happen, so...