Do as we do for MALLOC_PERTURB and set a sensible value for both ASAN_OPTIONS
and UBSAN_OPTIONS to abort on failure and give more helpful output at the
same time. We do not set these options if the user has exported a value
themselves to allow override.
In the last week alone, I've observed two cases where people were expecting
sanitizers to abort on failure and were surprised when it didn't:
1) 252d693797
2) c47df433f7
Correct this - which is in-line with meson's DWIM/DTRT philosophy.
Signed-off-by: Sam James <sam@gentoo.org>