respect OPENBLAS_{{CC, FC, HOSTCC}} env vars on linux

[mike-kfed]

this should fix issue #101

together with PR rust-ndarray/ndarray-linalg#354
I can run this successfully on a x86_64 machine to cross compile for a Raspi 4

export OPENBLAS_CC=aarch64-linux-gnu-gcc
export OPENBLAS_FC=aarch64-linux-gnu-gfortran
export OPENBLAS_HOSTCC=gcc
export OPENBLAS_TARGET=ARMV8 # CORTEXA72
cargo build --release --target=aarch64-unknown-linux-gnu

[Dirreke]

As OpenMathLib/OpenBLAS#4011, sometimes, we also need to add RANLIB=ranlib to cross compile on some systems.

Could you help add OPENBLAS_RANLIB to the list?

[mike-kfed]

I added OPENBLAS_RANLIB now, but you @Dirreke will have to test it, as ranlib is not my usecase. please let me know if that works for you ^^

[Dirreke]

It works! Thanks！

[Dirreke]

I think it will be great if we add some default parameter to this. Would you like to do this? Or would you like to give me auth to edit this PR?

[mike-kfed]

I don't think that there is a meaningful default for cross compiling, the build script cannot read the devs mind which target to pick. And for compiling onto the host platform this crate works fine.

[Dirreke]

Maybe we can get the parameters by cc.
On the other hand, you are right. It's enough for crossing compile

[Dirreke]

@mike-kfed Do you have time to rebase this PR?
@termoshtt Would you like to mearge this PR? It can really solve the cross-compile issue on many platforms. Thanks