meson: Add an option to specify the WGL gallium megadriver filename

Specifying the name at build time, as opposed to renaming after
the build, serves two purposes:
1. The link from Mesa's OpenGL32.dll and (and EGL/GLES) to the
   megadriver is done by filename. If using these frontends, the
   megadriver can't be renamed afterwards. And Windows doesn't
   have very good symlink support, so that's not really an option
   either.
2. The symbol (PDB) filename is also embedded in the DLL using the
   build-time expected filename. Renaming can produce odd artifacts
   while debugging.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7115
Reviewed-by: Bill Kristiansen <billkris@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18239>

meson_options.txt

@@ -163,6 +163,12 @@ option(
   value : 'disabled',
   description : 'build gallium "clover" OpenCL frontend.',
 )
+option(
+  'gallium-windows-dll-name',
+  type : 'string',
+  value : 'libgallium_wgl',
+  description : 'name of gallium megadriver DLL built for Windows. defaults to libgallium_wgl.dll to match DRI',
+)
 option(
   'opencl-spirv',
   type : 'boolean',

src/gallium/targets/wgl/meson.build

@@ -30,8 +30,9 @@ wgl_def = custom_target(
              '--compiler_id', cc.get_argument_syntax(), '--cpu_family', host_machine.cpu_family()]
 )
 
+gallium_wgl_name = get_option('gallium-windows-dll-name')
 libgallium_wgl = shared_library(
-  'gallium_wgl',
+  gallium_wgl_name,
   ['wgl.c'],
   vs_module_defs : wgl_def,
   include_directories : [
@@ -46,7 +47,7 @@ libgallium_wgl = shared_library(
     driver_d3d12, driver_zink, winsys_d3d12_wgl
   ],
   link_args : gallium_wgl_link_args,
-  name_prefix : 'lib',
+  name_prefix : '',
   install : true,
 )
 libgallium_wgl_build_dir = meson.current_build_dir()