Reported on RH 7.2 and 8.0. Compilation failed because system headers that
needed _GNU_SOURCE on these Redhat versions got included before it was
defined. To ensure _GNU_SOURCE is defined, added an autodetect for it to
config1.h and removed the need to set it in unixres.c. __linux__ is also
checked elsewhere.
- incorporate a more portable way of calling the real getres*id() functions
on Linux platforms that uses the glibc interface rather than calling
the system call directly. The previous version didn't work on ia64 linux.
