Including C headers inside of `extern "C"` breaks use from C++. Hoist
the includes of standard C headers above the block so we don't try
to mangle the stdlib.
I initially tried to scope this with a targeted change but it's too
hard to do correctly that way. This way, we avoid whack-a-mole.
Firefox is working around this in their e21461b7b8b39cc31ba53c47d4f6f310c673ff2f
commit.
Bug: https://bugzilla.mozilla.org/1953080
dlopen() does not set errno on failure, rather you're supposed to call
dlerror() to get the latest error. dlerror() return a string so
instead return -ENOENT from weakjack_load_by_path().
Depending on errno weakjack_load() could think it successfully loaded
the library, and later module-jack-tunnel would crash because it call
a NULL function pointer.
JACK current_msec can be in MONOTONIC_RAW or MONOTONIC, depending on how
JACK was compiled (but it's likely MONOTONIC_RAW). PipeWire requires the
nsec field to be in MONOTONIC so take some time snapshots from both
clocks and apply a translation.
Also make sure we only get the nsec time from streams that exist.
See #3886
This way we can search for the real jack library instead of using our
custom libjack.so. Add an option to override the search path with
LIBJACK_PATH and an option to select the library name to load.
