Makefile: enable ThinLTO with clang on Windows, use lld when linking clang builds

lld works with GCC builds too, but you have to disable LTO if you want to use it.

Common.mak

@@ -238,7 +238,10 @@ ifeq ($(findstring clang,$(CLANG_POTENTIAL_VERSION)),clang)
     endif
 endif
 
+LLD := 0
+
 ifneq (0,$(CLANG))
+    override LLD := 1
     CLANGXXNAME := $(subst clang,clang++,$(CLANGNAME))
     override CC := $(CLANGNAME) -x c
     override CXX := $(CLANGXXNAME) -x c++
@@ -431,11 +434,6 @@ else
     LTO := 1
 endif
 
-ifneq (0,$(CLANG))
-    ifeq ($(PLATFORM),WINDOWS)
-        LTO := 0
-    endif
-endif
 ifeq (0,$(CLANG))
     ifeq (0,$(GCC_PREREQ_4))
         override LTO := 0
@@ -595,9 +593,16 @@ endif
 
 ifneq (0,$(LTO))
     COMPILERFLAGS += -DUSING_LTO
-    COMMONFLAGS += -flto
+    ifeq (0,$(CLANG))
+        COMMONFLAGS += -flto
+    else
+        COMMONFLAGS += -flto=thin
+    endif
 endif
 
+ifeq (1,$(LLD))
+    COMMONFLAGS += -fuse-ld=lld
+endif
 
 ##### Debugging