Question

My makefile:

ifndef VEC_LEN
        VEC_LEN = 1
endif

my_target: a.c
        gcc a.c -DVEC_LEN=$(VEC_LEN)

Is there a way to tell make that my_target should be updated when VEC_LEN changes?

Update:

My scripts now look like this (and they work):
Makefile

SHELL := /bin/bash

# Define the answer if not defined yet
ANSWERTOLIFETHEUNIVERSEANDEVERYTHING ?= 42

# Update the header file if the answer has changed
# := always executes the shell command, = does not! Quote from http://www.gnu.org/software/make/manual/make.html:
#      immediate = deferred
#      immediate := immediate
DUMMY := $(shell ./updateAnswer.sh $(ANSWERTOLIFETHEUNIVERSEANDEVERYTHING) >logMakefile.txt)

answer : answer.h 
    echo "Updated!"
    touch answer

updateAnswer.sh

#!/bin/bash

# Check if the definition of the answer has changed in the header file
# If yes, re-write it. If not, do not touch it to avoid an updated timestamp.
if grep -q "ANSWER ${1}" answer.h
then
    echo "ANSWER unchanged, is still ${1}."
else
    echo "#define ANSWER ${1}" >answer.h
    echo 'Answer has changed:'
    cat answer.h
fi

Example output:

simon@x220:~$ make
echo "Updated!"
Updated!
touch answer
simon@x220:~$ make
make: `answer' is up to date.
simon@x220:~$ make ANSWERTOLIFETHEUNIVERSEANDEVERYTHING=3
echo "Updated!"
Updated!
touch answer
simon@x220:~$ make ANSWERTOLIFETHEUNIVERSEANDEVERYTHING=3
make: `answer' is up to date.

Answer 1

Assuming that the language is C,
I think the most straightforward way is probably:

1. Prepares vec_len.h which has a line #define VEC_LEN 1
2. Adds #include "vec_len.h" in a.c
3. If VEC_LEN‘s value has to be updated, rewrites vec_len.h
4. Builds using usual .c file and header file dependency

EDIT:
Though this is a little naive way, does the following change work in your
situation?

Prepares a script(define.sh) like the following:

#!/usr/bin/bash
echo '#define VEC_LEN' $1 > vec_len.h

And adds the following lines at the beginning of the makefile:

VEC_LEN ?= 1
DUMMY := $(shell define.sh $(VEC_LEN))