98c38dc6d6
sets C99 as standard.
strdup() is not part of C99.
For now set `-D_POSIX_C_SOURCE=200809L` macro to have strdup() in C99.
Using `gnu99` instead would be another option.
We should take more care to use glib functions whenever possible.
Regards https://github.com/profanity-im/profanity/issues/1357
Found this when looking to fix bug https://github.com/profanity-im/profanity/issues/1357
Not sure if it is related.
man 3 getenv sais:
```
As typically implemented, getenv() returns a pointer to a string within
the environment list. The caller must take care not to modify this
string, since that would change the environment of the process.
```
