feat(build): use lld for clang if available

Also simplify the CI.

closes https://github.com/official-stockfish/Stockfish/pull/6592

No functional change

.github/workflows/avx2_compilers.yml

@@ -74,7 +74,6 @@ jobs:
         run: |
           export CXXFLAGS="-Werror"
           if [ "${{ matrix.ver }}" -ge 20 ]; then
-            export LDFLAGS="-fuse-ld=lld"
             apt install -y lld
           fi
           make clean

src/Makefile

@@ -854,27 +854,40 @@ endif
 ### This is a mix of compile and link time options because the lto link phase
 ### needs access to the optimization flags.
 ifeq ($(optimize),yes)
-ifeq ($(debug), no)
+ifeq ($(debug),no)
+	ifneq ($(KERNEL),Darwin)
+		LLD_BIN := $(shell command -v ld.lld 2>/dev/null)
+		ifeq ($(LLD_BIN),)
+			LLD_BIN := $(shell command -v lld 2>/dev/null)
+		endif
+		ifneq ($(LLD_BIN),)
+			ifeq ($(comp),clang)
+				LDFLAGS += -fuse-ld=lld
+			else ifeq ($(comp),gcc)
+				ifneq ($(gccisclang),)
+					LDFLAGS += -fuse-ld=lld
+				endif
+			endif
+		endif
+	endif
+
 	ifeq ($(comp),$(filter $(comp),clang icx))
 		CXXFLAGS += -flto=full
 		ifeq ($(comp),icx)
 			CXXFLAGS += -fwhole-program-vtables
-                endif
-		ifeq ($(target_windows),yes)
-			CXXFLAGS += -fuse-ld=lld
 		endif
 		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 -flto-partition=one
-		LDFLAGS += $(CXXFLAGS) -flto=jobserver
-	else
-		CXXFLAGS += -flto=full
-		LDFLAGS += $(CXXFLAGS)
-	endif
+		ifeq ($(gccisclang),)
+			CXXFLAGS += -flto -flto-partition=one
+			LDFLAGS += $(CXXFLAGS) -flto=jobserver
+		else
+			CXXFLAGS += -flto=full
+			LDFLAGS += $(CXXFLAGS)
+		endif
 
 # To use LTO and static linking on Windows,
 # the tool chain requires gcc version 10.1 or later.