gdb: Default to _WIN32_WINNT=0x0600

This commit pulls in the GDB patch that sets `_WIN32_WINNT` to 0x0600
when not explicitly defined in the CFLAGS.

Note that, libstdc++, when building with the win32 thread model, only
supports std::mutex and std:condition_variable if _WIN32_WINNT >=
0x0600.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio