Fix subversion17 with neon causing SIGSEGV on OS X 10.10 Yosemite

By the following steps, `git-svn` cause SIGSEGV on http access.

* `brew edit git`, change the dependency to `subversion17` from `subversion`.
* `brew install git-svn --with-brewed-svn`
* `git svn clone http://...` or `git svn rebase` or `git svn fetch`

results in:

```
error: git-svn died of signal 11
```


This pull request enables neon version check and `ra_neon` will not be built when neon is 0.30.0.
`brew install -v subversion17 --perl` warns this at `configure`.

```
:
checking neon library version... 0.30.0
You have neon version 0.30.0,
but Subversion needs neon 0.29.6.

An appropriate version of neon could not be found, so libsvn_ra_neon
will not be built.  If you want to build libsvn_ra_neon, please
install neon 0.29.6 on this system.

no suitable neon found
:
```

http access is still alive because `ra_serf` is still enabled.