use -std=gnu++11 under cygwin & msys2
use -std=gnu++11 under cygwin & msys2

this is a quick fix for these targets using the path added in:
9f23fbc Fix compilation with MinGW32

the side effect is that it will enable newer POSIX functions like fdopen
/ rand_r. setting _POSIX_C_SOURCE directly might be a more robust
solution as suggested in https://crbug.com/webm/1708.

Change-Id: I9ada1de847ca9bd23f0972b5ad8ba425efe08f68

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm