Add preliminary support of targeting Wasm

This builds on the work on portable-intrinsics branch:

- define WEBP_ENABLE_WASM_SIMD to enable WebAssembly optimizations, this
defines a constant (WEBP_USE_WASM_SIMD) that we can use in the future to
enable Wasm SIMD intrinsics
- build dwebp with JS build
- add a placeholder dec_wasm.c file, we don't do any special
initialization so it will fall back to c functions
- add Wasm to cpu checks, it needs to come before all checks since the
Emscripten toolchain will emulate x86 systems

Change-Id: I12de720304ff19fff82c8d100defbc60353787a9

cmake/config.h.in

@@ -108,6 +108,9 @@
 /* Set to 1 if JPEG library is installed */
 #cmakedefine WEBP_HAVE_JPEG 1
 
+/* Set to 1 if Wasm SIMD is supported */
+#cmakedefine WEBP_HAVE_WASM_SIMD
+
 /* Set to 1 if NEON is supported */
 #cmakedefine WEBP_HAVE_NEON