Makefile: Always use SED variable

[t-b]

We need to have a proper sed, even on MacOSX. So let's use the variable
introduced in 191aa0f (:wrench: overworked maintaner targets,
2019-03-28) in more places.

[coveralls]

Coverage remained the same at 100.0% when pulling 88adffd on t-b:feature/use-proper-SED-in-makefile into cf74131 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!