ext_drmingw: Downgrade dbghelp.dll and etc.
The dbghelp.dll, dbgcore.dll and symserv.dll included with the DrMingw 0.9.3 release comes from Debugging Tools 10.0.19041.685, which uses the api-ms-win-downlevel-kernel32-l2-1-0.dll contract not available on Windows 7 systems by default.
DrMingw 0.9.2 still came with the ones from 10.0.17763.1, which should in theory still work on Windows 7 systems. Since we build DrMingw from source for the mingw-w64 build and use the binary DrMingw releases only for the Debugging Tools DLLs, this should not affect its functionality.
** Important Notice ** : Windows 7 is not officially supported.
See also: https://github.com/jrfonseca/drmingw/issues/55, https://github.com/jrfonseca/drmingw/commit/7f8a701861f4aaf4c58250308bb99c5717ac1876
cc @lsegovia