abuild.conf: define format-security and int-conversion errors

format-security warns of usage such as `printf(x)`, which is usually a
security hole.

int-conversion is very useful to find cases such as

```
error: assignment to 'const char *' from 'int' makes pointer from integer without a cast [-Werror=int-conversion]
 msg = strerror_r(errnum, buf, buflen);
```

where the usage of things like the wrong strerror_r are legitimate
errors in the application that cause it to crash. it makes more sense
for the compiler to reject it instead, and this does that.

abuild.conf

@@ -1,4 +1,4 @@
-export CFLAGS="-Os -fomit-frame-pointer"
+export CFLAGS="-Os -fomit-frame-pointer -Werror=format-security -Werror=int-conversion"
 export CXXFLAGS="$CFLAGS"
 export CPPFLAGS="$CFLAGS"
 export LDFLAGS="-Wl,--as-needed,-O1,--sort-common"