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.

meson.build

@@ -978,10 +978,7 @@ endif
 
 #####################################################################
 
-threads = dependency('threads')
-librt = cc.find_library('rt')
 libm = cc.find_library('m')
-libdl = cc.find_library('dl')
 
 # Header presence check only — dgettext itself is resolved via dlopen_libintl() at runtime, so we never
 # link against libintl. On glibc dgettext lives in libc; on musl gettext-dev provides libintl.h alongside
@@ -1758,9 +1755,7 @@ libsystemd = shared_library(
         link_with : [libc_wrapper_static,
                      libbasic_static],
         link_whole : [libsystemd_static],
-        dependencies : [librt,
-                        threads,
-                        userspace],
+        dependencies : [userspace],
         link_depends : libsystemd_sym,
         install : true,
         install_tag: 'libsystemd',
@@ -1779,14 +1774,11 @@ if static_libsystemd != 'false'
                 install_tag: 'libsystemd',
                 install_dir : libdir,
                 pic : static_libsystemd_pic,
-                dependencies : [libdl,
-                                libgcrypt_cflags,
+                dependencies : [libgcrypt_cflags,
                                 liblz4_cflags,
                                 libm,
-                                librt,
                                 libxz_cflags,
                                 libzstd_cflags,
-                                threads,
                                 userspace],
                 c_args : libsystemd_c_args + (static_libsystemd_pic ? [] : ['-fno-PIC']))
 
@@ -1804,8 +1796,7 @@ libudev = shared_library(
                      '-Wl,--version-script=' + libudev_sym_path],
         link_with : [libsystemd_static],
         link_whole : libudev_basic,
-        dependencies : [threads,
-                        userspace],
+        dependencies : [userspace],
         link_depends : libudev_sym,
         install : true,
         install_tag: 'libudev',
@@ -1922,10 +1913,6 @@ nss_template = {
                 libshared_static,
                 libbasic_static,
         ],
-        'dependencies' : [
-                librt,
-                threads,
-        ],
         'install' : true,
         'install_tag' : 'nss',
         'install_dir' : libdir,
@@ -1938,10 +1925,7 @@ pam_template = {
                 libsystemd_static,
                 libshared_static,
         ],
-        'dependencies' : [
-                libpam_cflags,
-                threads,
-        ],
+        'dependencies' : libpam_cflags,
         'install' : true,
         'install_tag' : 'pam',
         'install_dir' : pamlibdir,