Before we had the following scheme:
mempool_enabled() would check mempool_use_allowed, and
libsystemd-shared would be linked with a .c file that provides mempool_use_allowed=true,
while other things would linked with a different .c file with mempool_use_allowed=false.
In the new scheme, mempool_enabled() itself is a weak symbol. If it's
not found, we assume false. So it only needs to be provided for libsystemd-shared,
where it can return false or true.
test-set-disable-mempool is libshared, so it gets the symbol. But then we
actually disable the mempool via envvar. mempool_enable() is called to check
its return value directly.
Mempool use is enabled or disabled based on the mempool_use_allowed symbol that
is linked in.
Should fix assert crashes in external programs caused by #9792.
Replaces #10286.
v2:
- use two different source files instead of a gcc constructor
Daan De Meyer
Zbigniew Jędrzejewski-Szmek
Yu Watanabe