Makefile: really generate glibc locales in parallel

Makefile: really generate glibc locales in parallel

To generate the glibc locale data, we call into a recursive Makefile,
so as to generate locales in parallel. This is done as part of a
target-finalize hook.

However, that hook is registered after all packages have been parsed,
and as such, it maye be registered after hooks defined in packages.

Furthermore, the expansion of target-finalize hooks is done in a recipe,
so it is not easy to understand whether this generates a "simple" rule
or not.

As a consequence, despite the use of $(MAKE), make may not notice that
the command is a recursive call, and will decide to close the jobserver
file-descriptors, yielding warnings like:
    make[2]: warning: jobserver unavailable: using -j1.  Add '+' to
    parent make rule.

This causes the lcoale data to not be generated in parallel, which is
initially all the fuss about using a sub-makefile...

So, do as suggested, and prepend the hook with a '+', so that it is
explicit to make that it should not close its jobserver fds.

Fixes: 6fbdf5159607 (Makefile: Parallelize glibc locale generation)
Signed-off-by: Yann E. MORIN <[email protected]>
Cc: Gleb Mazovetskiy <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>