Add Compile-time checking of LOG arguments

[geekymoose]

Description

Logger use printf like format with variables.
For instance LOG_DEBUG(0, "Variable a = %d", varA) with varA an integer.
In case of user gives wrong format (For instance %s), a segfault is generated.

Issue

No nice error message displayed!
It is really hard from the end user point of view to understand the error comes from its log format. (And anyway, logger should be able to segfault the program like this.)

Compiler-time format checking is required. (For the vsnprintf call)

Reproduce the error

Run this beautiful piece of code

#include <logger/elephant.h>

int main(int argc, char** argv) {
    elephant::init();

    int counter = 20;
    LOG_DEBUG(0, "Counter %s", counter); // %s should be %d -> segfault!
    return 0;
}

To run, you can use
g++ -g -Wall -I/path/to/logger/includes main.cpp /path/to/elephantlib/libelephant.a -pthread -lstdc++fs
(lstdc++fs is for the required experimental/filesystem)

Last words

Little elephant wish you good luck!

Your code:

The (only..) message you get