Parallelize Link Time Optimization for GCC, CLANG and MINGW

This patch tries to run multiple LTO threads in parallel, speeding up
the build process of optimized builds if the -j make parameter is used.
This mitigates the longer linking times of optimized builds since the
integration of the NNUE code. Roughly 2x build speedup.

I've tried a similar patch some two years ago but it ran into trouble
with old compiler versions then. Since we're on the C++17 standard now
these old compilers should be obsolete.

closes official-stockfish/Stockfish#2943

No functional change.

src/Makefile

@@ -282,6 +282,9 @@ ifeq ($(COMP),gcc)
 	ifneq ($(KERNEL),Darwin)
 	   LDFLAGS += -Wl,--no-as-needed
 	endif
+
+	gccversion = $(shell $(CXX) --version)
+	gccisclang = $(findstring clang,$(gccversion))
 endif
 
 ifeq ($(COMP),mingw)
@@ -496,18 +499,28 @@ endif
 ### needs access to the optimization flags.
 ifeq ($(optimize),yes)
 ifeq ($(debug), no)
-	ifeq ($(comp),$(filter $(comp),gcc clang))
+	ifeq ($(comp),clang)
+		CXXFLAGS += -flto=thin
+		LDFLAGS += $(CXXFLAGS)
+
+# GCC and CLANG use different methods for parallelizing LTO and CLANG pretends to be
+# GCC on some systems.
+	else ifeq ($(comp),gcc)
+	ifeq ($(gccisclang),)
 		CXXFLAGS += -flto
+		LDFLAGS += $(CXXFLAGS) -flto=jobserver
+	else
+		CXXFLAGS += -flto=thin
 		LDFLAGS += $(CXXFLAGS)
 	endif
 
 # To use LTO and static linking on windows, the tool chain requires a recent gcc:
 # gcc version 10.1 in msys2 or TDM-GCC version 9.2 are know to work, older might not.
 # So, only enable it for a cross from Linux by default.
-	ifeq ($(comp),mingw)
+	else ifeq ($(comp),mingw)
 	ifeq ($(KERNEL),Linux)
 		CXXFLAGS += -flto
-		LDFLAGS += $(CXXFLAGS)
+		LDFLAGS += $(CXXFLAGS) -flto=jobserver
 	endif
 	endif
 endif
@@ -693,7 +706,7 @@ config-sanity:
 	@test "$(comp)" = "gcc" || test "$(comp)" = "icc" || test "$(comp)" = "mingw" || test "$(comp)" = "clang"
 
 $(EXE): $(OBJS)
-	$(CXX) -o $@ $(OBJS) $(LDFLAGS)
+	+$(CXX) -o $@ $(OBJS) $(LDFLAGS)
 
 clang-profile-make:
 	$(MAKE) ARCH=$(ARCH) COMP=$(COMP) \