Mark variables declared at global scope in headers as 'inline'.

[bangerth]

Summary of Changes

const variables declared at global scope have internal linkage, and so if they are declared in a header file, they are replicated in every file that #include the header. They also can't be the target of using declarations that exports the variable into a C++20 module. This change marks a number of variables with inline, which gives these variables external linkage, allows the linker to unify all of these variables between different .o files, and also allows for their use in C++20 modules.

There is a separate question whether these variables could or should be constexpr instead of just const. The difference between constexpr and const has no effect on linkage, and so in the spirit of taking one step at a time, this patch does not address the constexpr vs const question.

This patch fixes #8871. Related to dealii/dealii#18071.

Release Management

• Affected package(s): 5.6.1
• Issue(s) solved (if any): Let global variables have external linkage. #8871
• License and copyright ownership: Same license as used throughout CGAL.

[sloriot]

Successfully tested in CGAL-6.1-Ic-155

note that constexpr are added by #7966