meson: Move libdl_dep to the top level

Instead of letting each directory to find its way to link with libdl,
it is easier to put the check in the top level, so its result can be
used by all directories.

It is a follow-up of https://gitlab.gnome.org/GNOME/glib/merge_requests/810.

gio/tests/meson.build

@@ -140,17 +140,13 @@ if host_machine.system() != 'windows'
   }
 
   if have_rtld_next
-    # FIXME: This list will probably grow; see
-    # https://gitlab.gnome.org/GNOME/glib/issues/1739
-    no_libdl_systems = ['freebsd', 'netbsd', 'openbsd']
-
     gio_tests += {
       'gsocketclient-slow' : {
         'depends' : [
           shared_library('slow-connect-preload',
             'slow-connect-preload.c',
             name_prefix : '',
-            dependencies: cc.find_library('dl', required: not no_libdl_systems.contains(host_machine.system())),
+            dependencies: libdl_dep,
             install_dir : installed_tests_execdir,
             install: installed_tests_enabled,
           )

gmodule/meson.build

@@ -4,23 +4,8 @@ g_module_need_uscore = 0
 g_module_broken_rtld_global = 0
 g_module_have_dlerror = 0
 
-libdl_dep = [ ]
-g_module_lib_args = [ ]
 g_module_impl = ''
 
-dlopen_dlsym_test_code = '''
-#include <dlfcn.h>
-int glib_underscore_test (void) { return 42; }
-int main (int argc, char ** argv) {
-  void *f1 = (void*)0, *f2 = (void*)0, *handle;
-  handle = dlopen ((void*)0, 0);
-  if (handle) {
-    f1 = dlsym (handle, "glib_underscore_test");
-    f2 = dlsym (handle, "_glib_underscore_test");
-  }
-  return (!f2 || f1);
-}'''
-
 # On Windows force native WIN32 shared lib loader
 if host_system == 'windows'
   g_module_impl = 'G_MODULE_IMPL_WIN32'
@@ -28,16 +13,12 @@ if host_system == 'windows'
 # dlopen() filepath must be of the form /path/libname.a(libname.so)
 elif host_system == 'aix'
   g_module_impl = 'G_MODULE_IMPL_AR'
-elif cc.links(dlopen_dlsym_test_code, name : 'dlopen() and dlsym() in system libraries')
-  g_module_impl = 'G_MODULE_IMPL_DL'
 # NSLinkModule (dyld) in system libraries (Darwin)
 elif cc.has_function('NSLinkModule')
   g_module_impl = 'G_MODULE_IMPL_DYLD'
   g_module_need_uscore = 1
-elif cc.links(dlopen_dlsym_test_code, args : '-ldl', name : 'dlopen() and dlsym() in libdl')
+elif have_dlopen_dlsym
   g_module_impl = 'G_MODULE_IMPL_DL'
-  libdl_dep = cc.find_library('dl')
-  g_module_lib_args = '-ldl'
 endif
 
 # additional checks for G_MODULE_IMPL_DL
@@ -51,7 +32,7 @@ if g_module_impl == 'G_MODULE_IMPL_DL'
   elif meson.has_exe_wrapper()
     # FIXME: communicate result via stdout instead of return value, so non-0 return is not printed in bold red
     rres = cc.run(dlopen_dlsym_test_code,
-                  args : g_module_lib_args,
+                  dependencies : libdl_dep,
                   name : 'dlsym() preceding underscores')
     if host_system == 'windows' or rres.returncode() == 0
       g_module_need_uscore = 1
@@ -61,7 +42,7 @@ if g_module_impl == 'G_MODULE_IMPL_DL'
     g_module_need_uscore = 0
   endif
 
-  if cc.has_function('dlerror', args : g_module_lib_args)
+  if cc.has_function('dlerror', dependencies : libdl_dep)
     g_module_have_dlerror = 1
   endif
 endif

meson.build

@@ -721,6 +721,28 @@ elif cc.links(clock_gettime_test_code, args : '-lrt', name : 'clock_gettime in l
   librt = cc.find_library('rt')
 endif
 
+dlopen_dlsym_test_code = '''
+#include <dlfcn.h>
+int glib_underscore_test (void) { return 42; }
+int main (int argc, char ** argv) {
+  void *f1 = (void*)0, *f2 = (void*)0, *handle;
+  handle = dlopen ((void*)0, 0);
+  if (handle) {
+    f1 = dlsym (handle, "glib_underscore_test");
+    f2 = dlsym (handle, "_glib_underscore_test");
+  }
+  return (!f2 || f1);
+}'''
+libdl_dep = []
+if cc.links(dlopen_dlsym_test_code, name : 'dlopen() and dlsym() in system libraries')
+  have_dlopen_dlsym = true
+elif cc.links(dlopen_dlsym_test_code, args : '-ldl', name : 'dlopen() and dlsym() in libdl')
+  have_dlopen_dlsym = true
+  libdl_dep = cc.find_library('dl')
+else
+  have_dlopen_dlsym = false
+endif
+
 # if statfs() takes 2 arguments (Posix) or 4 (Solaris)
 if have_func_statfs
   if cc.compiles(glib_conf_prefix + '''