It works fine for me now.
Many, many thanks for your quick help.

I might be too impatient but I still don't see the changeset on savannah...

My hg id is 0a887758d1cc.
That's the most recent I can see here:
  http://hg.savannah.gnu.org/hgweb/octave/shortlog/
(note that I am compiling in the source tree).

Many thanks for this.

I haven't been able to test it yet as I have compilation problems with the latest version:

making main.cc from main.in.cc
g++ -DHAVE_CONFIG_H -I. -I..  -I../liboctave/array -I../liboctave/cruft/misc -I../liboctave/numeric -I../liboctave/system -I../liboctave/util -I../libinterp -I../libinterp/corefcn -I../libinterp/corefcn -I../libgnu -I../libgnu    -Wall -W -Wshadow -Wold-style-cast -Wformat -Wpointer-arith -Wwrite-strings -Wcast-align -Wcast-qual -g -O2 -pthread -MT octave-main.o -MD -MP -MF .deps/octave-main.Tpo -c -o octave-main.o `test -f 'main.cc' || echo './'`main.cc
main.cc: In function ‘int main(int, char**)’:
main.cc:419:11: error: ‘gui_libs’ was not declared in this scope
main.cc:432:11: error: ‘start_gui’ was not declared in this scope
main.cc:444:7: error: ‘gui_libs’ was not declared in this scope
main.cc:444:19: error: ‘start_gui’ was not declared in this scope

It is related to this:
https://mailman.cae.wisc.edu/pipermail/octave-maintainers/2013-October/037125.html
but the changeset does not fix it for me.
I compile with --disable-gui.

I have a MEX file that works fine on MATLAB but not with Octave. I managed to minimise the code to illustrate the problem, see teststruct.c attached.

After compilation, with MATLAB:

>> teststruct(struct('t',5))
ans =
    t: 10

while with Octave:

octave> teststruct(struct('t',5))
ans =
  11508528x0 struct array containing the fields:
    t

If any of the commented line is used instead of the dynamic allocation then it runs fine.

My apologies if the problem is with my code and not Octave but I am puzzled because of the difference of behaviour between Octave and MATLAB.