    % scons -Q
    cc -o file1.o -c -I. file1.c
    cc -o file2.o -c -I. file2.c
    cc -o file3.o -c -I. file3.c
    cc -o prog file1.o file2.o file3.o
    %     [CHANGE THE CONTENTS OF hello.h]
    % scons -Q --debug=explain
    scons: rebuilding `file1.o' because `hello.h' changed
    cc -o file1.o -c -I. file1.c
    scons: rebuilding `file3.o' because `hello.h' changed
    cc -o file3.o -c -I. file3.c
    scons: rebuilding `prog' because:
               `file1.o' changed
               `file3.o' changed
    cc -o prog file1.o file2.o file3.o