Use the python3 that configure found

When running preproc.py, use the python3 that the configure script found
instead of assuming (via preproc.py's #! line) that it is called
"python3" and is located in PATH. This allows the user to specify a
different python3 by running e.g.:

./configure ac_cv_path_PYTHON3=/path/to/python3

scripts/configure

@@ -5381,7 +5381,7 @@ if test "x${PYTHON3}" == "xno"; then
    fi
 
 else
-   MCPP="\${MAGICDIR}/scripts/preproc.py -ccomm"
+   MCPP="${PYTHON3} \${MAGICDIR}/scripts/preproc.py -ccomm"
    MSED="cat"
    usingPython3=1
 fi

scripts/configure.in

@@ -306,7 +306,7 @@ if test "x${PYTHON3}" == "xno"; then
    fi
 
 else
-   MCPP="\${MAGICDIR}/scripts/preproc.py -ccomm"
+   MCPP="${PYTHON3} \${MAGICDIR}/scripts/preproc.py -ccomm"
    MSED="cat"
    usingPython3=1
 fi