Link the system glslang instead of the bundled wrap.

R80 added runtime GLSL to SPIR-V compilation and takes glslang from a wrap-git
subproject unconditionally, so configure fails without network access. Every
other dependency here (zimg, libp2p, vulkan) already goes through
dependency(), so only glslang needs this.

Only glslang's C interface is used (glslang_c_interface.h and
resource_limits_c.h), which dev-util/glslang provides in libglslang and
libglslang-default-resource-limits. The wrap pins the same vulkan-sdk release
that dev-util/glslang tracks, so the accepted dialect matches what upstream
intends.

--- a/meson.build
+++ b/meson.build
@@ -183,10 +183,16 @@
     dependency('zimg', version: '>=3.0.5'),
 ]

-# Runtime GLSL to SPIR-V compilation: glslang is embedded statically and always present,
-# taken from the wrap rather than the system so the accepted dialect stays a pinned
-# platform property instead of following whatever glslang the machine happens to have.
-glslang_dep = subproject('glslang').get_variable('glslang_dep')
+# Runtime GLSL to SPIR-V compilation. Upstream embeds glslang from the wrap to
+# pin the accepted dialect, but that means downloading it at build time. Link
+# the system glslang instead; only the C interface is used, which lives in
+# libglslang plus libglslang-default-resource-limits.
+glslang_dep = declare_dependency(
+    dependencies: [
+        cxx.find_library('glslang'),
+        cxx.find_library('glslang-default-resource-limits'),
+    ],
+)

 soversion = is_win ? '' : run_command(
     'grep',