Call AC_COMPILE_IFELSE correctly in RSXS m4 script

This silences the last block of warnings in RSXS bootstrap. Newer gnulib
enforces calling AC_COMPILE_IFELSE with the AC_LANG_SOURCE macro wrapping
the source to be compiled. See (1).

(1) http://lists.gnu.org/archive/html/bug-autoconf/2010-09/msg00091.html
Update RSXS gnulib m4 macros

This fixes a large body of warnings due to dependency order changes
with newer gnulib. See (1).

(1)
http://lists.gnu.org/archive/html/bug-autoconf/2007-10/msg00017.html