meson: drop libdl, threads, and librt dependencies

Our baseline glibc is 2.34, which merged libdl, libpthread (the
dependency('threads') target), and librt into libc. Empty .so/.a stubs
remain for backward compatibility with old binaries, but new builds
resolve dl_*, pthread_*, mq_*, timer_*, etc. directly from libc.
On musl the same libraries are likewise empty stubs.

Drop the libdl, threads, and librt entries from every meson.build, and
remove the now-stale 'Libs.private: -lrt -pthread' from libudev.pc.in
since both flags resolve to empty link-time stubs on glibc 2.34+ and
musl.

Verified with readelf -d that libsystemd.so, libudev.so, and systemd no
longer carry DT_NEEDED entries for libdl/libpthread/librt.

src/basic/meson.build

@@ -212,15 +212,12 @@ libbasic_static = static_library(
         include_directories : basic_includes,
         implicit_include_directories : false,
         dependencies : [libbzip2_cflags,
-                        libdl,
                         libgcrypt_cflags,
                         liblz4_cflags,
                         libm,
-                        librt,
                         libxz_cflags,
                         libz_cflags,
                         libzstd_cflags,
-                        threads,
                         userspace],
         c_args : ['-fvisibility=default'],
         build_by_default : false)