pybind: silence gcc warnings about '-Wstrict-prototypes'

pybind: silence gcc warnings about '-Wstrict-prototypes'

this silences following warning

```
warning: cc1plus: command line option ‘-Wstrict-prototypes’ is valid for
C/ObjC but not for C++ [enabled by default]
```

* distutils pass "-DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes" to
  compiler by default if OPT environment variable is not set. so we opt to
  remove '-Wstrict-prototypes' as we are using c++ as the language of the
  generated bindinging.
* also use "env" to pass the environment variables to avoid unnecessary
  quote added by distutils, which just breaks the generated CLI command
  line.

Signed-off-by: Kefu Chai <kchai@redhat.com>