Update `llvm-config` checking on Linux

Makefile

@@ -15,7 +15,15 @@ ifeq ($(OS), Darwin)
 	LDFLAGS:=$(LDFLAGS) -lLLVM-C
 endif
 ifeq ($(OS), Linux)
-	LLVM_CONFIG=llvm-config-11
+	ifneq ($(shell command -v llvm-config-11),)
+	    LLVM_CONFIG=llvm-config-11
+	else
+		ifneq ($(shell llvm-config --version | grep 11),)
+			LLVM_CONFIG=llvm-config
+		else
+			$(error "Requirement: llvm-config must be version 11")
+		endif
+	endif
 
 	CFLAGS:=$(CFLAGS) $(shell $(LLVM_CONFIG) --cxxflags --ldflags)
 	LDFLAGS:=$(LDFLAGS) $(shell $(LLVM_CONFIG) --libs core native --system-libs)