Move AM_INIT_AUTOMAKE above AC_PROG_CC

Compiling on Cygwin x64 gives following warning:

```
...
checking whether build environment is sane... yes
/bin/sh: /home/ljani/missing: No such file or directory
configure: WARNING: 'missing' script is too old or missing
checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
...
```

This PR fixes the problem by moving `AM_INIT_AUTOMAKE` above `AC_PROG_CC`.

Warning: this PR is only tested on Cygwin, but afaik this is the correct order. For example, the [amhello's `configure.ac`](https://www.gnu.org/software/automake/manual/html_node/amhello_0027s-configure_002eac-Setup-Explained.html) uses this order.