[Bug-prolog] spaces in gprolog build directory

[ernie cohen]

The makefile of EnginePL passes directories to cpp_headers as a simple
space-separated string.
This causes the build to break if done from a directory with spaces (not
uncommon for windows
users).

This can either be fixed in obvious ways, or you can just give a warning
somewhere.

thanks,
ernie

PS gprolog is great.