Rerun if PYO3_CROSS env variable changes.

[jameshilliard]

Looks like I missed this in #1514.

[messense]

Thanks!

[davidhewitt]

I must have missed this from #1514 - what's the purpose of the PYO3_CROSS env var? It's not documented; it looks like #1514 introduced this, but it doesn't do anything special except force cross compilation. However at least PYO3_CROSS_LIB_DIR is also required at the same time, so it doesn't seem that there's a point to having a separate PYO3_CROSS variable...

[jameshilliard]

I must have missed this from #1514 - what's the purpose of the PYO3_CROSS env var?

Oh, I think I was using that so that I could ensure pyo3 was running in cross compilation mode for cases where PYO3_CROSS_LIB_DIR is getting set in a different part of the build system. There's basically a chain of build systems when I cross compile, buildroot being the meta build system which invokes the setuptools build system which then invokes pyo3 using the cargo build system. So I had added that so at any point cross compilation can be turned on from any build system essentially to ensure one gets a useful error message in the event something in the build chain doesn't set PYO3_CROSS_LIB_DIR. Also added it so that we have a way to force cross compilation without affecting anything else in the event PYO3_CROSS_LIB_DIR gets removed/changed.