Use CMake's built-in toolchain detection

[jkoritzinsky]

Try removing manual resolution of toolchain for non-cross builds and let CMake resolve the toolchain itself.

[jkotas]

@janvorli understands the cmake plumbing

[janvorli]

I've looked why we have added the stuff that is now being removed. It comes from the ancient #35 and it looks like it was fixing a problem related to building with clang 3.4 or 3.5.
But I want to verify that it always picks the same version of the llvm-xxx tools as the clang. I have many llvm versions installed on my Linux box side by side, so I can do that.

[jkoritzinsky]

@janvorli sounds good. If you can take a look at that when you have some free cycles that'd be great!

[janvorli]

Unfortunately, the results of my testing show this change is problematic. With your change, the cmake variables are set as follows for all versions of clang:

CMAKE_AR: /usr/bin/ar
CMAKE_LINKER: /usr/bin/ld
CMAKE_NM: /usr/bin/nm
CMAKE_OBJDUMP /usr/bin/objdump

Without your change, the correct llvm versions of the tools are used. Example - clang 4.0:

CMAKE_AR: /usr/bin/llvm-ar-4.0
CMAKE_LINKER: /usr/bin/llvm-link-4.0
CMAKE_NM: /usr/bin/llvm-nm-4.0
CMAKE_OBJDUMP /usr/bin/llvm-objdump-4.0

[jkoritzinsky]

Ah well. Guess this isn’t gonna work without some additional work to preserve the behavior. I’ll close this for now and take another stab at it some time.

[janvorli]

It causes the same issue for cross build. For example alpine arm32 build

CMAKE_AR: /usr/bin/armv6-alpine-linux-musleabihf-ar
CMAKE_LINKER: /usr/bin/armv6-alpine-linux-musleabihf-ld
CMAKE_NM: /usr/bin/armv6-alpine-linux-musleabihf-nm
CMAKE_OBJDUMP /usr/bin/armv6-alpine-linux-musleabihf-objdump

while the state before your change was the same as for x64.