Add interface library for project-wide compile options

[daljit46]

Previously we relied on add_compile_options and add_compile_definitions for defining compiler definitions and options for the entire project. This commit replaces that behaviour by defining a new mrtrix::common INTERFACE library that links to all targets in the project (the library is linked to mrtrix::core). This is more CMake-idiomatic and prevents the propagation of compiler definitions to targets that do not link to mrtrix::core (useful when compiling MRtrix3 as a subproject of another project). Ideally, we should be using CMake presets for this kind of stuff, but we need to support CMake 3.16.

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"