My problem is in fact that mingw32-gcc.exe --version
tells version 3.4.2, which apparently does not support @file option
(Well, just by comparing the same help section between version 3.4.2 and 4.2.0 I can see this difference : http://gcc.gnu.org/onlinedocs/gcc-3.4.2/gcc/Overall-Options.html#Overall-Options (http://gcc.gnu.org/onlinedocs/gcc-3.4.2/gcc/Overall-Options.html#Overall-Options)
http://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Overall-Options.html#Overall-Options (http://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Overall-Options.html#Overall-Options)
)
Just to add that the GCC documentation http://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Invoking-G_002b_002b.html#Invoking-G_002b_002b (http://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Invoking-G_002b_002b.html#Invoking-G_002b_002b) tells that most options passed to gcc can be also passed to g++. So, because the @file option is neither listed in the section concerning options controlling the C++ dialect nor in the section concerning options controlling the C dialect, it is quite logical to assume that it is either supported by both g++ and gcc, or not supported by any of them.
BR,
Vincent.
PS: I have tested in command line the @file option with mingw32-gcc.exe and it does not work either.