meson: if dep_dl is an empty list, it's not a dependency object

It's ok to use an empty list for dependencies:, but it's not ok to try to
use the found() method of it.

See also https://github.com/mesonbuild/meson/issues/2324

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/meson.build b/meson.build
index e846759..1c42007 100644
--- a/meson.build
+++ b/meson.build
@@ -922,7 +922,7 @@
 if dep_m.found()
   gl_priv_libs += '-lm'
 endif
-if dep_dl.found()
+if dep_dl != [] and dep_dl.found()
   gl_priv_libs += '-ldl'
 endif