Cross-compilation support

[mgorny]

So we definitely need some hacks to make build systems more helpful for cross-compiling.

Live TODO:

• support overriding include directory
• support overriding library directory
• support overriding SOABI and EXT_SUFFIX
• support overriding sysconfig.get_platform()
  • needed for correct wheel names
• support for wrapping setuptools (or arbitrary tool) invocations (i.e. esetup.py)
• Gentoo eclass support

[mgorny]

Ok, so my current idea is to use only --sysroot, and have gpep517 find sysconfig data there and get the remaining values out of it.

[thesamesam]

I think this is done now.

[hendrikdonner]

Additionally to SOABI and EXT_SUFFIX, CFLAGS and LDFLAGS should be overriden. Especially CFLAGS can contain flags like --march=armv7-a. You want the correct value from the sysroot.