meson: fix gallium-osmesa to build for windows

v2: - set so_version to '' (only affects windows)
    - always set lib prefix to 'lib', even on msvc
v5: - key NO_EXPORTS on shared glapi instead of gles.

Acked-by: Kristian H. Kristensen <hoegsberg@google.com>
diff --git a/meson.build b/meson.build
index 6657de4..8c5ae52 100644
--- a/meson.build
+++ b/meson.build
@@ -1450,7 +1450,11 @@
   if with_osmesa == 'gallium' and not with_gallium_softpipe
     error('OSMesa gallium requires gallium softpipe or llvmpipe.')
   endif
-  osmesa_lib_name = 'OSMesa'
+  if host_machine.system() == 'windows'
+    osmesa_lib_name = 'osmesa'
+  else
+    osmesa_lib_name = 'OSMesa'
+  endif
   osmesa_bits = get_option('osmesa-bits')
   if osmesa_bits != '8'
     if with_dri or with_glx != 'disabled'