Introduce a build folder

.gitignore

@@ -0,0 +1 @@
+build/

build.sh

@@ -2,6 +2,7 @@
 
 set -xe
 
-clang -Wall -Wextra -o twice twice.c -lm
-clang -Wall -Wextra -o gates gates.c -lm
-clang -Wall -Wextra -o xor xor.c -lm
+mkdir -p build/
+clang -Wall -Wextra -o build/twice twice.c -lm
+clang -Wall -Wextra -o build/gates gates.c -lm
+clang -Wall -Wextra -o build/xor xor.c -lm