Pass PIE flags for compiling on Android L
Fix minor compilation issues on CentOS.

- ensure size_t is defined when compiling xprintf.h
- declare overlay_route_record as a union field in overlay_route_record to avoid alignment issues
- set the result of fwrite and fputs
- support slightly older git version
radio works now

It could made an improvement to avoid collitions. Unfortunately the
radios start to transmit all after the where interupted be a
transmitting node.