Debian buster installs gcc-mingw-w64 8.3, causing compilation to
fail due to the hard-coded path for includes.

```
In file included from /usr/x86_64-w64-mingw32/include/minwindef.h:163,
                 from /usr/x86_64-w64-mingw32/include/windef.h:8,
                 from /usr/x86_64-w64-mingw32/include/windows.h:69,
                 from containerutility.h:3,
                 from argumentstream.cpp:1:
/usr/x86_64-w64-mingw32/include/winnt.h:1554:11: fatal error: x86intrin.h: No such file or directory
 # include <x86intrin.h>
           ^~~~~~~~~~~~~
compilation terminated.
```

This patch removes the hard-coded paths to consider for includes, and instead
enables the standard include paths by removing the `-nostdinc` option.

With this option removed, the following paths are considered on Debian buster:

```
x86_64-w64-mingw32-g++ -print-search-dirs
install: /usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/
programs: =/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/../../../../x86_64-w64-mingw32/bin/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/../../../../x86_64-w64-mingw32/bin/
libraries: =/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/../../../../x86_64-w64-mingw32/lib/x86_64-w64-mingw32/8.3-win32/:/usr/lib/gcc/x86_64-w64-mingw32/8.3-win32/../../../../x86_64-w64-mingw32/lib/
```

And compilation succeeds without having to manually specify paths:

```
x86_64-w64-mingw32-g++ -fno-exceptions -std=c++11 -static -DUNICODE -municode -O3 -s -o containerutility.exe argumentstream.cpp commands.cpp containerutility.cpp identity.cpp version.cpp
echo $?
0
```

There might have been a specific reason for hard-coding the paths and excluding
the standard paths, but no history could be found for this.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>