build: Define the preload variable name and separator globally

We may need this in multiple tests, so let's share the value

gio/tests/meson.build

@@ -331,11 +331,11 @@ if host_machine.system() != 'windows'
       'gsocketclient-slow' : {
         'depends' : slow_connect_preload_lib,
         'env' : {
-          'LD_PRELOAD': slow_connect_preload_lib.full_path(),
+          glib_exec_var_preload: slow_connect_preload_lib.full_path(),
         } + asan_env,
         'installed_tests_env' : {
         } + asan_env,
-          'LD_PRELOAD': installed_tests_execdir /
+          glib_exec_var_preload: installed_tests_execdir /
             fs.name(slow_connect_preload_lib.full_path()),
       },
     }

glib/tests/meson.build

@@ -266,12 +266,6 @@ else
                                       install_tag : 'tests',
                                       install: installed_tests_enabled)
 
-    if host_system not in ['ios', 'darwin']
-      var_preload = 'LD_PRELOAD'
-    else
-      var_preload = 'DYLD_INSERT_LIBRARIES'
-    endif
-
     asan_env = {}
     if 'address' in glib_sanitizers
       asan_env = {'ASAN_OPTIONS': 'verify_asan_link_order=0'}
@@ -281,10 +275,10 @@ else
       'gutils-user-database' : {
         'depends' : getpwuid_preload,
         'env' : {
-          var_preload: getpwuid_preload.full_path()
+          glib_exec_var_preload: getpwuid_preload.full_path()
         } + asan_env,
         'installed_tests_env' : {
-          var_preload: installed_tests_execdir / fs.name(getpwuid_preload.full_path())
+          glib_exec_var_preload: installed_tests_execdir / fs.name(getpwuid_preload.full_path())
         } + asan_env,
       },
     }

meson.build

@@ -2669,6 +2669,14 @@ gir_args = [
   '--quiet',
 ]
 
+if host_system in ['ios', 'darwin']
+  glib_exec_var_preload = 'DYLD_INSERT_LIBRARIES'
+  glib_exec_var_preload_separator = ':'
+else
+  glib_exec_var_preload = 'LD_PRELOAD'
+  glib_exec_var_preload_separator = ' '
+endif
+
 pkg = import('pkgconfig')
 windows = import('windows')
 gnome = import('gnome')