installation: configure script and target paths

Hi,

The configure script assumes that the Asterisk modules are located in PREFIX/lib. However, on some systems such as Gentoo Linux /lib, /lib64, /usr/lib and /usr/lib64 are all different directories with different content (no symlinks as in the olden days).
Also, the following assumes the PREFIX variable will be used for both the conf file and the modules which is usually not the case:

```
ASTERISK_MODDIR="${prefix}/lib/asterisk/modules"
ASTERISK_CONF_DIR="${prefix}/etc/asterisk"
```

To make a long story short I want to be able to install the Asterisk module in /usr/lib64/asterisk/modules and the conf file in /etc/asterisk.