Build fails on v5.2+

[geofft]

Since torvalds/linux@cdd750b, the __c_flags variable used by kernel-cflags-helper is gone, and we should just use _c_flags. Probably the right thing to do is use __c_flags if it exists and fall back to _c_flags, for correctness with older kernels. (You can also do version comparisons with like, $(shell expr) or something but that sounds like a pain.)

Maybe it's worth seeing if there's a cleaner way of doing all this.

Originally reported in lizhuohua/linux-kernel-module-rust#1.