meson: reuse libsystemd_static for libsystemd

This gets rid of recompilation, making things faster and avoids duplicated warnings.
The result seems to be the same:
$ ls -l build/libsystemd.so.0.20.0 build2/libsystemd.so.0.20.0
-rwxrwxr-x. 1 zbyszek zbyszek 3071312 Dec 19 11:45 build2/libsystemd.so.0.20.0
-rwxrwxr-x. 1 zbyszek zbyszek 3071760 Dec 19 11:11 build/libsystemd.so.0.20.0
$ diff -U1 <(objdump -T build/libsystemd.so.0.20.0|sed -r 's/[0-9a-f]{16}/________________/g') <(objdump -T build2/libsystemd.so.0.20.0|sed -r 's/[0-9a-f]{16}/________________/g')
-build/libsystemd.so.0.20.0:     file format elf64-x86-64
+build2/libsystemd.so.0.20.0:     file format elf64-x86-64

meson.build

@@ -1258,13 +1258,13 @@ libjournal_core = static_library(
 libsystemd_sym_path = '@0@/@1@'.format(meson.current_source_dir(), libsystemd_sym)
 libsystemd = shared_library(
         'systemd',
-        libsystemd_sources,
         journal_internal_sources,
         version : libsystemd_version,
         include_directories : includes,
         link_args : ['-shared',
                      '-Wl,--version-script=' + libsystemd_sym_path],
-        link_with : [libbasic],
+        link_with : libbasic,
+        link_whole : libsystemd_static,
         dependencies : [threads,
                         libgcrypt,
                         librt,