On at least OpenBSD, malloc(3) is preferred to other implementations
like jemalloc for various reasons.
It is common for portable third-party software (ports) to provide a
switch for en/disabling jemalloc.
tdesktop makes this easy since it does not contain any (je)malloc
specific code (anymore).
Introduce a new default-off option that disables jemalloc when enabled
explicitly; this also allow developers and package maintainers more
easily to try different malloc implementations, e.g. for debugging.
Memory leaks reported in the slightly old issue
"The app consumes an unexpectedly large amount of RAM"
https://github.com/telegramdesktop/tdesktop/issues/16084 could not
be reproduced on OpenBSD/amd64 with malloc(3) using tdesktop >= 3.3.1.
