meson: build libsystemd-core via an intermediate static library

By itself, this is not useful. I'm making this a separate commit to
make debugging easier. It turns out that meson does static libraries
using references, so the "static library" a tiny stub stub that refers
to the object files on disk and this has negligible cost:
$ ls -lhd build/src/core/libsystemd-core-257.{a,so}
-rw-r--r-- 1 zbyszek zbyszek  36K Jul  3 16:54 build/src/core/libsystemd-core-257.a
-rwxr-xr-x 1 zbyszek zbyszek 6.1M Jul  3 16:54 build/src/core/libsystemd-core-257.so

(cherry picked from commit d0689ee5fb)

src/core/meson.build

@@ -110,17 +110,13 @@ load_fragment_gperf_nulstr_c = custom_target(
 
 libcore_name = 'systemd-core-@0@'.format(shared_lib_tag)
 
-libcore = shared_library(
+libcore_static = static_library(
         libcore_name,
         libcore_sources,
         load_fragment_gperf_c,
         load_fragment_gperf_nulstr_c,
         include_directories : includes,
         c_args : ['-fvisibility=default'],
-        link_args : ['-shared',
-                     '-Wl,--version-script=' + libshared_sym_path],
-        link_depends : libshared_sym_path,
-        link_with : libshared,
         dependencies : [libacl,
                         libapparmor,
                         libaudit,
@@ -135,6 +131,16 @@ libcore = shared_library(
                         libselinux,
                         threads,
                         userspace],
+        build_by_default : false)
+
+libcore = shared_library(
+        libcore_name,
+        c_args : ['-fvisibility=default'],
+        link_args : ['-shared',
+                     '-Wl,--version-script=' + libshared_sym_path],
+        link_depends : libshared_sym_path,
+        link_whole: libcore_static,
+        link_with : libshared,
         install : true,
         install_dir : pkglibdir)