ICE failures with old GCC versions on the buildbots

[John W. Eaton]

I'm seeing internal compiler failures with GCC 4.9 and 5 on the buildbots:

GCC 4.9:

../../src/init2.c:52: MPFR assertion failed: p >= 2 && p <= 
((mpfr_prec_t)((mpfr_uprec_t)(~(mpfr_uprec_t)0)>>1))
../src/libinterp/operators/op-scm-scm.cc: In function ‘MSparse<T> 
quotient(const MSparse<T>&, const MSparse<T>&) [with T = 
std::complex<double>]’:
../src/libinterp/operators/op-scm-scm.cc:253:1: internal compiler error: 
Aborted
 }
 ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-4.9/README.Bugs> for instructions.
Preprocessed source stored into /tmp/ccPw5ALA.out file, please attach 
this to your bugreport.

GCC 5:

../../src/init2.c:52: MPFR assertion failed: p >= 2 && p <= 
((mpfr_prec_t)((mpfr_uprec_t)(~(mpfr_uprec_t)0)>>1))
In file included from ../src/liboctave/array/MSparse.h:123:0,
                 from ../src/liboctave/array/MatrixType.h:29,
                 from ../src/liboctave/array/dMatrix.h:31,
                 from ../src/liboctave/array/Range.h:30,
                 from ../src/libinterp/octave-value/ov.h:36,
                 from ../src/libinterp/octave-value/ovl.h:36,
                 from ../src/libinterp/operators/op-scm-scm.cc:29:
../src/liboctave/array/MSparse.cc: In function ‘MSparse<T> 
quotient(const MSparse<T>&, const MSparse<T>&) [with T = 
std::complex<double>]’:
../src/liboctave/array/MSparse.cc:478:1: internal compiler error: Aborted
 quotient (const MSparse<T>& a, const MSparse<T>& b)
 ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-5/README.Bugs> for instructions.


These failures seem to have started happening after I upgraded my 
systems that are running the buildbot workers so it could be caused by 
that and not a change in Octave.

It may be this bug:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=888422

I can investigate and see whether there is a workaround, but I'm 
wondering whether we should care about supporting builds with these 
compilers.  GCC 5 was first released approximately 3 years ago.

jwe