Problems with mod_auth_token installation - c

Question

TOOLS
apache2 -v : Server version: Apache/2.4.7 (Ubuntu)
ubuntu 14.04 LTS
I'd like to install mod_auth_token to secure some of my files but I'm having problems with the first command I need to run:
Command
./configure
Error message
configure: error: APXS not found in $PATH, please use with-apxs to specify the location of the apxs binary
Except that locate apxs returns none so I guess it's nowhere and
echo $PATH -> /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
Any ideas ?
UPDATE1
Found it, apxs2 was missing, to add it apt-get install apache2-dev
But now when I run make or make check or make install I get a warning and the installation stops:
mod_auth_token.c:65:3: warning: missing sentinel in funcgtion call [ -Wformat=]
ap_set_string_slot(cmd, config, apr_pstrcat(cmd->pool, arg, '/'));
^
Code
if (arg[len -1] != '/') {
/*here*/ ap_set_string_slot(cmd, config, apr_pstrcat(cmd->pool, arg, '/'));
conf->prefix_len = len + 1;
....

Answer 1

I was having this issue and developer here figured this for me (thanks Bill!).
According to Apache Docs https://apr.apache.org/docs/apr/1.6/group__apr__strings.html#ga7bd80c95ffb7b3f96bc78e7b5b5b0045 "The final string must be NULL"
He changed that line to:
ap_set_string_slot(cmd, config, apr_pstrcat(cmd->pool, arg, '/', NULL));
and it compiled.