Add JavaScript platform support (emcc, wasm).

Includes update to `README.md` with instructions on how to build a
GDNative library for webassembly.

SConstruct

@@ -86,7 +86,7 @@ opts.Add(EnumVariable(
     'platform',
     'Target platform',
     host_platform,
-    allowed_values=('linux', 'freebsd', 'osx', 'windows', 'android', 'ios'),
+    allowed_values=('linux', 'freebsd', 'osx', 'windows', 'android', 'ios', 'javascript'),
     ignorecase=2
 ))
 opts.Add(EnumVariable(
@@ -375,6 +375,34 @@ elif env['platform'] == 'android':
     env.Append(CCFLAGS=['--target=' + arch_info['target'] + env['android_api_level'], '-march=' + arch_info['march'], '-fPIC'])#, '-fPIE', '-fno-addrsig', '-Oz'])
     env.Append(CCFLAGS=arch_info['ccflags'])
 
+elif env["platform"] == "javascript":
+    env["ENV"] = os.environ
+    env["CC"] = "emcc"
+    env["CXX"] = "em++"
+    env["AR"] = "emar"
+    env["RANLIB"] = "emranlib"
+    env.Append(CPPFLAGS=["-s", "SIDE_MODULE=1"])
+    env.Append(LINKFLAGS=["-s", "SIDE_MODULE=1"])
+    env["SHOBJSUFFIX"] = ".bc"
+    env["SHLIBSUFFIX"] = ".wasm"
+    # Use TempFileMunge since some AR invocations are too long for cmd.exe.
+    # Use POSIX-style paths, required with TempFileMunge.
+    env["ARCOM_POSIX"] = env["ARCOM"].replace("$TARGET", "$TARGET.posix").replace("$SOURCES", "$SOURCES.posix")
+    env["ARCOM"] = "${TEMPFILE(ARCOM_POSIX)}"
+
+    # All intermediate files are just LLVM bitcode.
+    env["OBJPREFIX"] = ""
+    env["OBJSUFFIX"] = ".bc"
+    env["PROGPREFIX"] = ""
+    # Program() output consists of multiple files, so specify suffixes manually at builder.
+    env["PROGSUFFIX"] = ""
+    env["LIBPREFIX"] = "lib"
+    env["LIBSUFFIX"] = ".bc"
+    env["LIBPREFIXES"] = ["$LIBPREFIX"]
+    env["LIBSUFFIXES"] = ["$LIBSUFFIX"]
+    env.Replace(SHLINKFLAGS='$LINKFLAGS')
+    env.Replace(SHLINKFLAGS='$LINKFLAGS')
+
 env.Append(CPPPATH=[
     '.',
     env['headers_dir'],
@@ -413,6 +441,8 @@ if env['platform'] == 'android':
     arch_suffix = env['android_arch']
 if env['platform'] == 'ios':
     arch_suffix = env['ios_arch']
+if env['platform'] == 'javascript':
+    arch_suffix = 'wasm'
 
 library = env.StaticLibrary(
     target='bin/' + 'libgodot-cpp.{}.{}.{}{}'.format(