cmake defaults this value to 1GB, which should always work. The problem is with python support:  If it is enabled, the build takes a few hours (tested with parallel (4x) build on AMD FX(tm)-6300), and the user will only realize after a very long time that he can and probably should have intervened.  That's why I think bailing out is better, but I could change the ebuild that this is only done when python support is enabled.

I would have opted for epause+ebeep to give the user a chance to see what parameters can (and should) be set before the build starts, but since this is no longer supported, bailing out seemed to be the best option.