Difficulty building on alpine linux

Hi,

I have just tried to build mavproxy on alpine, eg:

```
apk update && apk add python-pip python-dev gcc libxml2 libxml2-dev py-lxml libxslt libxslt-dev musl-dev
pip install MAVproxy
```

But I run into errors about conflicting functions
```
mavnative/mavnative.c:322:13: error: conflicting types for '__assert_fail'
extern void __assert_fail(const char *__assertion, const char *__file, unsigned int __line, const char *__function)
^~~~~~~~~~~~~
In file included from mavnative/mavnative.c:14:0:
/usr/include/assert.h:19:16: note: previous declaration of '__assert_fail' was here
_Noreturn void __assert_fail (const char *, const char *, int, const char *);
^~~~~~~~~~~~~
error: command 'gcc' failed with exit status 1
```
Any ideas what I should do to build this?

Many thanks,
Ed

Start with mavnative/mavnative.c and compare its __assert_fail declaration with Alpine's /usr/include/assert.h. Reproduce the pip install MAVproxy build using the listed apk dependencies. Done means MAVProxy builds successfully on Alpine without the conflicting declaration.

Written by the indexing model from the issue text.