Clang baseline bump

Current master README.md has the following on which Clang versions are 
supported:

> * Linux:
>     * Runtime: RHEL 6 or CentOS 6
>     * Build: GCC 4.8.1 or Clang
[...]
> If you want to use Clang with the LibreOffice compiler plugins, the minimal
> version of Clang is 3.4. Since Xcode doesn't provide the compiler plugin
> headers, you have to compile your own Clang to use them on macOS.

However, I found that:

* Clang 3.4.2 no longer passes our configure checks, at least since 
<https://cgit.freedesktop.org/libreoffice/core/commit/?id=ae16870b2c84c116cde8279a47db106a4fa9d529> 
"Build as C++17 when GCC/Clang supports it" (which added a

  #pragma GCC diagnostic ignored "-Wpragmas"

that Clang 3.4 complains about, and which would need a preceding

  #pragma GCC diagnostic ignored "-Wunknown-pragmas"

to silence the Clang 3.4 warning, like is done in 
bin/gen-boost-headers).  Clang 3.5.0 passes the configure checks.

* Clang older than 3.8.0 (tested: 3.4.2, 3.5.0, 3.6.0, 3.7.0) fail `make 
comilerplugins`, for varying reasons.  Our plugin code uses LLVM/Clang 
functionality that was not available (or was available in different 
form) back then.

If there are no objections, I will do the following changes on master:

* note in README.md that Clang < 3.5 is known not to work to compile LO 
on Linux; and

* make the relevant changes so that --enable-compiler-plugins requires 
at least Clang 3.8.0.