Check for RTLD_NEXT

Check for RTLD_NEXT being present, and disable the gsocketclient-slow
test if it's absent, since the shlib dependency of that test requires
RTLD_NEXT to function.

This allows the testsuite to be built on Cygwin, which behaves
exactly like UNIX, but doesn't have RTLD_NEXT.

gio/tests/meson.build

@@ -136,6 +136,12 @@ if host_machine.system() != 'windows'
     'unix-mounts' : {},
     'unix-streams' : {},
     'g-file-info-filesystem-readonly' : {},
+    'gschema-compile' : {'install' : false},
+    'trash' : {},
+  }
+
+  if have_rtld_next
+    gio_tests += {
       'gsocketclient-slow' : {
         'depends' : [
           shared_library('slow-connect-preload',
@@ -153,9 +159,8 @@ if host_machine.system() != 'windows'
           'LD_PRELOAD': '@0@/slow-connect-preload.so'.format(installed_tests_execdir),
         },
       },
-    'gschema-compile' : {'install' : false},
-    'trash' : {},
     }
+  endif
 
   # Uninstalled because of the check-for-executable logic in DesktopAppInfo
   # unable to find the installed executable

meson.build

@@ -584,6 +584,12 @@ if cc.has_header_symbol('dlfcn.h', 'RTLD_GLOBAL')
   glib_conf.set('HAVE_RTLD_GLOBAL', 1)
 endif
 
+have_rtld_next = false
+if cc.has_header_symbol('dlfcn.h', 'RTLD_NEXT')
+  have_rtld_next = true
+  glib_conf.set('HAVE_RTLD_NEXT', 1)
+endif
+
 # Check whether to use statfs or statvfs
 # Some systems have both statfs and statvfs, pick the most "native" for these
 if have_func_statfs and have_func_statvfs