build issue: ld.lld

[haampie]

Describe the bug

Sometimes ld.lld can be located as an executable, but not used by the compiler: -fuse-ld=lld fails.

Locating happens here

mrtrix3/cmake/LinkerSetup.cmake

Line 11 in 025eb0f

find_program(LLVM_LINKER NAMES "ld.lld")

Setting the linker flag here:

mrtrix3/cmake/LinkerSetup.cmake

Lines 16 to 22 in 025eb0f

	if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.29)
	set(CMAKE_LINKER_TYPE "LLD" CACHE STRING "Linker type")
	else()
	set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fuse-ld=lld")
	set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fuse-ld=lld")
	set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -fuse-ld=lld")
	endif()

Two requests:

1. Please make this opt-in. lld may work well, but let it be a user choice instead of overriding toolchain defaults.
2. Test for -fuse-ld=lld with try-compile instead of searching for ld.lld the executable.

Platform/Environment/Version

Please provide the following information:

• OS: Ubuntu 22.04
• MRtrix3 version: dev branch

[daljit46]

The intention of using LLD when available was to enable fast (incremental) build times by default (especially for non-developers who want to build MRtrix3). I admit that overriding toolchain defaults is undesirable though and we should probably not mess with that. Maybe it's best to hide this behind a MRTRIX_USE_LLD option.
Ideally, even for other options such as MRTRIX_WARNING_AS_ERRORS we should probably rely on CMake presets instead of hardcoding compiler flags inside our CMakeLists.txt files. However, since 3.16 is our minimum CMake target version we cannot use them everywhere.

Test for -fuse-ld=lld with try-compile instead of searching for ld.lld the executable.

Yes, this also makes sense to me.

[daljit46]

Closed by #2989.