Don't set _POSIX_C_SOURCE for VXI/RPC related files. Make vxi.h the first #include in all affected files and #undef the _POSIX_C_SOURCE macro in vxi.h. This avoids various build issues on e.g. FreeBSD or Mac OS X where setting _POSIX_C_SOURCE leads to the unavailability of certain types such as u_long (as used in the VXI/RPC code).