meson: don't advertise TLS support if glx wasn't build with it

Fixes: a47c525f328 ("meson: build glx")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>
diff --git a/meson.build b/meson.build
index 661a01e..85a724f 100644
--- a/meson.build
+++ b/meson.build
@@ -427,8 +427,10 @@
 endif
 
 # Android uses emutls for versions <= P/28. For USE_ELF_TLS we need ELF TLS.
+use_elf_tls = false
 if not ['windows', 'freebsd'].contains(host_machine.system()) and (not with_platform_android or get_option('platform-sdk-version') >= 29)
   pre_args += '-DUSE_ELF_TLS'
+  use_elf_tls = true
 endif
 
 if with_glx != 'disabled'