Makefile: Fix libFuzzer target

-DCMAKE_BUILD_TYPE=Asan doesn't seem to work. Pass sanitizer flags
manually.

Makefile

@@ -89,9 +89,10 @@ afl:
 	    $(CMARK) $(CMARK_OPTS)
 
 libFuzzer:
-	CC=clang CXX=clang++ cmake -S . -B $(BUILDDIR) \
-	    -DCMAKE_BUILD_TYPE=Asan \
-	    -DCMARK_LIB_FUZZER=ON
+	CC=clang CXX=clang++ \
+	CFLAGS=-fsanitize=address LDFLAGS=-fsanitize=address \
+	    cmake -S . -B $(BUILDDIR) \
+	        -DCMARK_LIB_FUZZER=ON
 	cmake --build $(BUILDDIR)
 	mkdir -p fuzz/corpus
 	$(BUILDDIR)/fuzz/cmark-fuzz \