build: Enable introspection data when cross-compiling
build: Enable introspection data when cross-compiling

It's entirely possible to generate GObject Introspection data when
cross-compiling: you just need a wrapper around g-ir-scanner and
g-ir-compiler to run it under an appropriate architecture emulator.

We should use the existing options to disable introspection in order to
gate this feature, without trying to be exceedingly clever.

ebassi/graphene,ebassi/graphene