put header into a separate file myUtil.h in the directory include

put functions into a separate file (myUtil.cc)
add c++ headers and your own header

compile with
g++ -c myUtil.cc -o myUtil.o -Iinclude


compile main without the source functions included, and add the Object code
g++ myMain.cc myUtil.o

create a library with ar
g++ -c normalenVektorUtil.cc -o normalenVektorUtil.o -Iinclude
ar rcs libmyVektorUtil.a normalenVektorUtil.o

link the library
g++ normalenVektorMain.cc -Iinclude -Llib -lmyVektorUtil