Couple of fixes to allow SIMD on Emscripten

- Add `-msimd128` to flags to actually enable WebAssembly SIMD
   when performing SIMD detection. It's currently required in
   addition to `-msse*` / `-mfpu=neon` flags which only perform
   translation of corresponding intrinsics to Wasm SIMD ones.
   See a discussion at emscripten-core/emscripten#12714 for
   automating this and making easier in the future.
 - Remove compilation branch that prevented definitions of
   `WEBP_USE_SSE` and `WEBP_USE_NEON` on Emscripten even when
   SIMD support was detected at compile-time.
 - Add an implementation of `VP8GetCPUInfo` for Emscripten which
   uses static `WEBP_USE_*` flags to determine if a corresponding
   SIMD instruction is supported. This is because Wasm doesn't
   have proper feature detection (yet) and requires making separate
   build for SIMD version anyway.

Change-Id: I77592081b91fd0e4cbc9242f5600ce905184f506

CMakeLists.txt

@@ -3,7 +3,13 @@ cmake_minimum_required(VERSION 3.5)
 project(WebP C)
 
 # Options for coder / decoder executables.
-option(WEBP_ENABLE_SIMD "Enable any SIMD optimization." ON)
+if(NOT EMSCRIPTEN)
+  # Disable SIMD on Emscripten by default, as it's a new unstable Wasm feature.
+  # Users can still explicitly opt-in to make a SIMD-enabled build.
+  set(WEBP_ENABLE_SIMD_DEFAULT ON)
+endif()
+option(WEBP_ENABLE_SIMD "Enable any SIMD optimization."
+       ${WEBP_ENABLE_SIMD_DEFAULT})
 option(WEBP_BUILD_ANIM_UTILS "Build animation utilities." ON)
 option(WEBP_BUILD_CWEBP "Build the cwebp command line tool." ON)
 option(WEBP_BUILD_DWEBP "Build the dwebp command line tool." ON)
@@ -26,7 +32,6 @@ if(WIN32)
 endif()
 
 if(WEBP_BUILD_WEBP_JS)
-  set(WEBP_ENABLE_SIMD OFF)
   set(WEBP_BUILD_ANIM_UTILS OFF)
   set(WEBP_BUILD_CWEBP OFF)
   set(WEBP_BUILD_DWEBP OFF)