automake: display whether sleep supports fractional seconds as yes/no.

automake: display whether sleep supports fractional seconds as yes/no.

https://lists.gnu.org/archive/html/automake/2024-07/msg00003.html

* m4/sanity.m4 (_AM_SLEEP_FRACTIONAL_SECONDS): Set
am_cv_sleep_fractional_seconds to yes/no, not true/false,
for consistency with other results.
Also "quote" this and other $am_cv values.
(_AM_FILESYSTEM_TIMESTAMP_RESOLUTION): Test for yes/no.
* t/ax/test-defs.in: Set am_cv_sleep_fractional_seconds to 'no',
not 'false'.