[patch,libgfortran] Handle invalid command line in EXECUTE_COMMAND_LINE

[FX]

10-day *ping* for my 3 Fortran patches:

- Handle invalid command line in EXECUTE_COMMAND_LINE (https://gcc.gnu.org/ml/gcc-patches/2015-08/msg00758.html)
- Use libbacktrace in libgfortran (https://gcc.gnu.org/ml/gcc-patches/2015-08/msg00762.html)
- Fix configure test for weakref support (https://gcc.gnu.org/ml/gcc-patches/2015-08/msg00796.html)