Availability test for XGAP

[olexandr-konovalov]

At the moment it only checks whether the -p command line option has been used:

xgap/PackageInfo.g

Line 93 in 0286846

AvailabilityTest := function() return GAPInfo.CommandLineOptions.p; end,

It will be useful to check the presence of the compiled XGAP binary as well.

[fingolfin]

I'll not implement this, but patches are welcome.

[RussWoodroofe]

But the suggested change may actually be counterindicated: for instance Gap.app also loads and uses the xgap library, even though the xgap binary may not be present.