Calling SED for a source in Makefile.am - sed

Question

I've got c++ code that needs a sed done to it prior to compilation. How do I place this into Makefile.am?
I tried the typical makefile setup and the target appears to not exist:
gentest.cc:
$(SED) -i "s|FIND|REPLACE|" gentest.cc
If you are interested as to why I want to do this, it's because I wrote my program (slider3.py) in python and my partner wrote his in c++ (gentest.cc) and his needs to call mine. I'm accomplishing this by editing the argv and then using execv().
... {
char **argv2 = new char*[argc];
memset(argv2,0,sizeof(argv2));
argv2[0] = "__PREFIX__/bin/slider3.py";
memcpy(argv2 + 1, argv + 2, sizeof(char *) * (argc - 2));
int oranges = execv(argv2[0], argv2);
printf("%s\n", strerror(oranges));
return oranges;
} ...
I've already handled getting the #! added to slider3.py and chmod +x by using the method that was not working for gentest.cc. I've also handled adding slider3.py to the list of files that get installed.
EXTRA_DIST=testite.sh slider3_base.py
bin_SCRIPTS = slider3.py
CLEANFILES = $(bin_SCRIPTS)
slider3.py: slider3_base.py
rm -f slider3.py
echo "#! " $(PYTHON) > slider3.py
cat slider3_base.py >> slider3.py
chmod +x slider3.py
gentest is defined this way in Makefile.am:
bin_PROGRAMS = gentest
gentest_SOURCES = gentest.cc
gentest_LDADD = libgen.a #../libsbsat.la $(LIBM)
And this fails to be run during make (note the # pattern is successfully expanded in Makefile):
gentest.cc:
$(SED) -i "s|__PREFIX__|#prefix#|" gentest.cc
Any ideas on how to get sed to run before compiling gentest.cc?

Answer 1

Don't use in-place sed.
Instead:
gentest_SOURCES = gentest-seded.cc
gentest-seded.cc : gentest.cc
$(SED) "s|__PREFIX__|#prefix#|" $< >$#

Answer 2

Have you ever considered #define-ing it in config.h (you're using autotools, right?) or passing it using -D when compiling? This is really not the case for sed.
The details from Andrew Y's answer:
in your C++ source, specify:
argv2[0] = SCRIPTPREFIX "/bin/slider3.py";
then compile with
-DSCRIPTPREFIX='"/your/script/prefix"'

Answer 3

Have you considered calling the Python code directly from the C++? Here is a tutorial on using boost to call python functions from C++. The method you are describing here seems very brittle.