meson: conditionally include -ldl in gbm pkg-config file

Follow libGL and only include -ldl in gbm pkg-config file if libdl was
actually found.  Many systems have these functions in libc and don't
have libdl.

Fixes: 816bf7d164 ("meson: build gbm")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>

meson.build

@@ -1853,6 +1853,12 @@ if dep_dl.found()
   gl_priv_libs += '-ldl'
 endif
 
+# FIXME: autotools lists this as incomplete
+gbm_priv_libs = []
+if dep_dl.found()
+  gbm_priv_libs += '-ldl'
+endif
+
 pkg = import('pkgconfig')
 
 if host_machine.system() == 'windows'

src/gbm/meson.build

@@ -64,7 +64,7 @@ pkg.generate(
   description : 'Mesa gbm library',
   version : meson.project_version(),
   libraries : libgbm,
-  libraries_private : '-ldl',  # FIXME: autotools lists this a incomplete
+  libraries_private : gbm_priv_libs,
 )
 
 if with_symbols_check