Add support for cross-compiling on Mac #186

[andrewgrant]

Adds support for cross-compiling to different targets when building on Mac.

E.g the following builds Opus for an arm64 Mac target (Apple silicon)

./configure --host=aarch64-apple-macos make && make install

This allows building Opus for both architectures and using lipo to create a universal binary.

[xnorpx]

@mark4o && @rillian for review?

Should neon be presumed for apple silicone?
Add config to .travis ci for coverage?

[silverbacknet]

NEON has been in all Apple silicon since 3GS, so it would be reasonable.

[andrewgrant]

Good point RE Neon. I'll add that and resubmit in the next day or so.

[rillian]

Can you please also document your rationale in the commit message, or a comment. I assume this is to work around autoconf's built-in cross-compiler detection not supporting Apple's new configuration, but it seems like there are several ways to do that. Why the separate clang_arch variables, for example, and why not set the flags directly in configure. That's the sort of thing someone having to maintain the changes later would need to know.

[j-schultz]

It's not part of this PR, but I found that the CMake build system is also doing dubious stuff here that pessimizes x64 builds on arm64:

opus/cmake/OpusConfig.cmake

Line 42 in ccaaffa

if(CMAKE_SYSTEM_PROCESSOR MATCHES "(i[0-9]86|x86|X86|amd64|AMD64|x86_64)")

From my understanding, CMAKE_SYSTEM_PROCESSOR matches arm64 here, causing the build system to check for NEON availability rather than SSE availability:

-- Looking for arm_neon.h
-- Looking for arm_neon.h - not found

The resulting binary is still compiled for x64, but without SIMD optimiations.

Edit: I think that's being tracked in this issue: #198

[xnorpx]

@j-schultz so the issue is cross compiling to x64 on a m1/m2 mac for cmake?

https://stackoverflow.com/questions/69803659/what-is-the-proper-way-to-build-for-macos-x86-64-using-cmake-on-apple-m1-arm

will try to test this

[j-schultz]

so the issue is cross compiling to x64 on a m1/m2 mac for cmake?

Correct. And probably the other way around, too (missing NEON optimizations when cross-compiling for arm64)

[xnorpx]

@j-schultz m1 stuff seems pretty hairy, unlikely I will get fixes in this release.

[charlifiiiii]

Is cross-compiling now supposed to work on Mac M1 ?
I tried to cross-compile for Mac Intel (x86_64) from a Mac M1 using this cmd
./configure --host=x86_64-apple-macos
and got the following error :
configure: error: no supported Get CPU Info method, please disable run-time CPU capabilities detection or intrinsics

Any advice ?
Thanks