Hotfix: various build system fixes to support CI

[ilumsden]

49 made UCX an optional dependency. However, it did not make the UCX-based code in dyad_dtl_init and dyad_dtl_finalize conditionally compiled. As a result, if a user built DYAD without --enable-ucx and then set the DTL mode to UCX, failures would occur due to dyad_dtl_init and dyad_dtl_finalize trying to call the corresponding UCX DTL functions, which would not have been built.

This hotfix PR fixes this small, but important, bug.

[hariharan-devarajan]

Also, fix configure.ac

PKG_CHECK_VAR([UCX_LIBDIR],
    [ucx >= 1.6.0],
    [libdir],
    [],
    [AC_MSG_FAILURE([Could not find libdir for UCX])]
)
AS_IF([test "x$UCX_LIBDIR" = "x"],
    [AC_MSG_FAILURE([check_var succeeded, but value is incorrect])]
)

[hariharan-devarajan]

Also can u merge it with the CI PR to see if its fixed and do any other changes required for that.

[ilumsden]

The scope of this PR has slightly changed. Now, it is a hotfix PR to address any build system bug that we encounter while testing #47.

[JaeseungYeom]

I will wait on the update for making sure to free dtl_handle in the finalization

[ilumsden]

@JaeseungYeom I've fixed the freeing of the DTL handle in dyad_dtl_finalize. I've also rebased my branch on the newest version of main.