meson.build: Propagate optimization flag for linking on Emscripten

Emscripten uses the optimization flag at the link time to enable
optimizations via Binaryen [1]. While meson.build currently recognizes the
-Doptimization option, it does not propagate it to the linking. This commit
updates meson.build to propagate the optimization flag to the linking when
targeting WebAssembly.

[1] https://emscripten.org/docs/optimizing/Optimizing-Code.html#how-emscripten-optimizes

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

Author: ktock

meson.build

@@ -878,6 +878,12 @@ elif host_os == 'openbsd'
     # Disable OpenBSD W^X if available
     emulator_link_args = cc.get_supported_link_arguments('-Wl,-z,wxneeded')
   endif
+elif host_os == 'emscripten'
+  # Emscripten uses the optimization flag also during the link time.
+  # https://emscripten.org/docs/optimizing/Optimizing-Code.html#how-emscripten-optimizes
+  if get_option('optimization') != 'plain'
+    emulator_link_args += ['-O' + get_option('optimization')]
+  endif
 endif
 
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