Issue #2221 - Enable link-time optimization for Spidermonkey by default if building it shared.

Currently only Windows can take advantage of this.

The reason why the arrays of deunified sources are named like that in
Spidermonkey's moz.build is to avoid UnsortedErrors in python. There's probably
a better way to handle that, but I just want something that works for now.

And yes, the deunified sources will be reunified if LTO is disabled.

modules/fdlibm/src/moz.build

@@ -38,6 +38,12 @@ if CONFIG['CC_TYPE'] == 'clang-cl':
         '-Wno-sign-compare', # signed/unsigned mismatch
     ]
 
+# Explicitly enable WPO and LTCG in MSVC if we're doing LTO.
+if CONFIG['JS_LTO']:
+    if CONFIG['_MSC_VER'] and not CONFIG['CLANG_CL']:
+        CFLAGS += [ '-GL' ]
+        CXXFLAGS += [ '-GL' ]
+
 SOURCES += [
     'e_acos.cpp',
     'e_acosh.cpp',