Require Clang to build Linux host binaries

[GoogleCodeExporter]

As of Mozc 2.16.2032.102, we are maintaining Mozc to be built with three 
compilers, Clang, GCC, and MSVC.

Here is the list of required compilers to build Mozc 2.16.2032.102:
- Android Host: GCC 4.6+
- Android Target: GCC 4.9 (default), Clang 3.5
- NaCl Host: GCC 4.6+
- NaCl Target: Clang 3.3
- Linux Host/Target: GCC 4.6+
- Mac Host/Target: Clang 3.3+
- Windows Host/Target: MSVC 2010, MSVC 2012, MSVC 2013

What we want to do here is to reduce the number of actively maintained build 
configurations.  Now we propose new compiler requirement as follows.
- Android Host: Clang 3.5+
- Android Target: GCC 4.9 (default), Clang 3.5
- NaCl Host: Clang 3.5+
- NaCl Target: Clang 3.3
- Linux Host/Target: Clang 3.5+
- Mac Host/Target: Clang 3.3+
- Windows Host/Target: MSVC 2010, MSVC 2012, MSVC 2013

Although GCC continues to be used to build binaries that need to run on Android 
devices, basically we are going to rely more on Clang rather than GCC in 
various aspects in foreseeable future.

Original issue reported on code.google.com by yukawa@google.com on 25 Jan 2015 at 7:30