meson: create libglsl declared dependency to propagate order-only deps

https://mesonbuild.com/FAQ.html#how-do-i-tell-meson-that-my-sources-use-generated-headers

A few locations had underspecified deps on the header files, and this
caused builds to fail given sufficient parallelism. Fix this by creating
an interface library that can be linked against, instead.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29115>

src/compiler/glsl/meson.build

@@ -232,12 +232,16 @@ files_libglsl_standalone = files(
   'standalone.h',
 )
 
+libglsl_headers = [
+  ir_expression_operation_h, cross_platform_settings_piece_all_h, bc1_glsl_h,
+  bc4_glsl_h, etc2_rgba_stitch_glsl_h, astc_glsl_h
+]
+
 libglsl = static_library(
   'glsl',
-  [files_libglsl, glsl_parser, glsl_lexer_cpp, ir_expression_operation_h,
+  [files_libglsl, glsl_parser, glsl_lexer_cpp, libglsl_headers,
    ir_expression_operation_strings_h, ir_expression_operation_constant_h,
-   float64_glsl_h, cross_platform_settings_piece_all_h, bc1_glsl_h, bc4_glsl_h,
-   etc2_rgba_stitch_glsl_h, astc_glsl_h],
+   float64_glsl_h],
   c_args : [c_msvc_compat_args, no_override_init_args],
   cpp_args : [cpp_msvc_compat_args],
   gnu_symbol_visibility : 'hidden',
@@ -247,6 +251,11 @@ libglsl = static_library(
   build_by_default : false,
 )
 
+idep_libglsl = declare_dependency(
+  sources: libglsl_headers,
+  link_with: libglsl,
+)
+
 libglsl_standalone = static_library(
   'glsl_standalone',
   [files_libglsl_standalone, ir_expression_operation_h],

src/mesa/meson.build

@@ -480,8 +480,7 @@ libmesa = static_library(
     inc_include, inc_src, inc_mapi, inc_mesa, inc_gallium, inc_gallium_aux,
     inc_libmesa_asm, include_directories('main'),
   ],
-  link_with : [libglsl, libmesa_sse41],
-  dependencies : [idep_nir, idep_vtn, dep_vdpau, idep_mesautil],
+  link_with : [libmesa_sse41],
+  dependencies : [idep_libglsl, idep_nir, idep_vtn, dep_vdpau, idep_mesautil],
   build_by_default : false,
 )
-