Recent addition of the GCC __attribute__(alloc_size) to some functions
in mem.h broke the build when using Clang 3.5. This commit introduces
configure-time checks for all GCC attributes used in the Serval source
code, and adapts accordingly.
- 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
