depcomp: remove support for SGI C/C++ compilers These compilers are only meant to run on IRIX, and that system has seen its last release in 2006, and is expected to lose support from SGI in December 2013: <http://www.sgi.com/services/support/irix_mips_support.html> This change fixes automake bug#12967 and bug#12978. * lib/depcomp (sgi): Remove this depmode. * NEWS: Update. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>