add --sysroot configuration option

[boegel]

This will come in useful when building certain software (GCC, binutils, ...) in an environment where you don't want to pick up anything from the host environment, for example when cross-compiling or when installing software on top of an alternative environment like Gentoo Prefix.

[casparvl]

Looks pretty straightforward to me. Just to clarify your ideas here: you don't envision the --sysroot option will do anything on the framework side I guess? I mean, I guess we mostly want to define this option in framework so that we can query its setting in the relevant EasyBlocks and customize behaviour accordingly there?

[boegel]

@casparvl Exactly, see easybuilders/easybuild-easyblocks#2142 (Perl) and easybuilders/easybuild-easyblocks#2143 (binutils) for example.

I'm not planning anything specific on the framework side for now, although it's possible we could enhance the sanity check when --sysroot is set, to make sure nothing outside the specified sysroot is picked up (by checking ldd output, etc.).

Whether that's a viable approach remains to be seen, but it could be interesting...

[casparvl]

I'm not planning anything specific on the framework side for now, although it's possible we could enhance the sanity check when --sysroot is set, to make sure nothing outside the specified sysroot is picked up (by checking ldd output, etc.).

Interesting idea. I guess we could try and see with a couple of ldd's if 'all' cases where it picks up something from outside the sysroot are 'wrong'. I mean, the things outside the sysroot it should pick up on are drivers and such, but those shouldn't typically show up with an ldd I guess... It IS very hard to otherwise find out that you're linking against stuff outside of your sysroot...

[casparvl]

LGTM

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @boegel!