systemd: switch to C.UTF-8 locale when building

systemd: switch to C.UTF-8 locale when building

When BR2_REPRODUCIBLE is enabled or host uses non UTF-8 capable locale
building systemd fails with an error:

[1/1080] Generating systemd.bg.catalog with a meson_exe.py custom command.
FAILED: catalog/systemd.bg.catalog
/buildroot/output/host/bin/python3 /buildroot/output/host/bin/meson --internal exe /buildroot/output/build/systemd-236/build/meson-private/meson_exe_sed_232a0623cc7ce2cd67ec72ed784b76307102ed76.dat
Warning: You are using 'ANSI_X3.4-1968' which is not a Unicode-compatible locale.
You might see errors if you use UTF-8 strings as filenames, as strings, or as file contents.
Please switch to a UTF-8 locale for your platform.
...
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 1079: ordinal not in range(128)
package/pkg-generic.mk:247: recipe for target '/buildroot/output/build/systemd-236/.stamp_built' failed
make: *** [/buildroot/output/build/systemd-236/.stamp_built] Error 1

This patch changes default host system locale from C to C.UTF-8 when
building systemd package to fix this issue. It also introduces
BR2_NEEDS_HOST_C_UTF8_LOCALE flag that checks if this locale is available on
the host system. If locale is not available error message is show and build
process is stopped.

Signed-off-by: Julius Kriukas <julius@kriukas.lt>
[Thomas: use C.UTF-8 instead of en_US.UTF-8.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>