Building library issue libudns - c

Question

First of all thanks for your attention and help.
I have been trying to ./configure and build a program that use a lot of libraries, and specifically libudns. I have been installing several libraries that were needed or by apt-get or by compile, and all of them works, but libudns (which is freaking me out).
This program make use of a configure script that include the following lines of code that add the flag -ludns to the Makefile:
if [ "x$WITH_UDNS" == "xy" ]; then
mkl_lib_check "udns" HAVE_UDNS fail CC "-ludns" \
"#include <udns.h>
void *f();void *f(){return dns_init;}"
fi
When I type ./configure the script checks if all libraries are presents on the system by pkg-config and by compile, as you can see in the following snap:
As you can see, configure cannot see this library.
What I have done is try to install this package by the two different ways:
By pkg-config: sudo apt-get install libudns-dev
By compile:
git clone https://github.com/shadowsocks/libudns
cd libudns
./autogen.sh
./configure && make
sudo make install
With this two ways, I have both:
/usr/lib/x86_64-linux-gnu/libudns.so
/usr/local/include/udns.h
These paths are the same where others libraries are installed, for example PostgreSQL, which you can that is detected:
/usr/lib/x86_64-linux-gnu/libpq.so
/usr/include/postgresql/libpq-fe.h
But the result is always the same:
Does anybody knows how to link this library?
Also, I have try these other things:
Copy udns.h to /usr/include: sudo cp /usr/local/include/udns.h /usr/include/udns.h
Make a sample program only to emulate this check:
#include <stdio.h>
#include <udns.h>
struct dns_ctx* ctx;
void *f();
void * f(){return dns_init;}
int main(int argc, char** argv){
int do_open = 0;
printf("Hola, mundo\n");
f(ctx, do_open);
}
And when I try to build this program with:
gcc main.c -o hello_world -ludns
it WORKS!?!
I have also try to build this program without the -ludns flag, and it gives me the same error as before:
So, I do not understand where is the fail, considering that as you can see in the second image, the -ludns flag is present.
Thanks a lot for your time. Any advise will be welcomed.