meson: Add build-lsfd option and make rt dependency optional

meson: Add build-lsfd option and make rt dependency optional

A dependency on the rt library is unnecessarily required when checking
for the clock_gettime and timer_create functions.
This causes the build to fail if the rt library is not found.
This should not fail the build as rt is only required for the checks.
Additionally, the lsfd executable and some tests require rt.
There is currently no option to toggle building lsfd.

This PR makes it possible to build without the rt library.
Function checks no longer require rt for the build.
The function checks for the rt library only run when rt is available.
This PR adds an option to allow building without lsfd.
This makes it possible to build without the executable that requires rt.
To not require rt for the test, a additional check has been added.
The effected tests won't be built unless rt has been found.

Signed-off-by: Jordan Williams <jordan@jwillikers.com>
(cherry picked from commit 06b27058a04c1ac9eba44f19e320ecd862f6d75f)