Enable RTTI when building our plugins.

LLVM prefers to disable RTTI.  They have their own approach to dynamic typing that they like better.  But we're already making heavy use of Boost, and some Boost features do need RTTI.  Hopefully these two big libraries can coexist peacefully.

Interestingly, this worked fine without forcing RTTI on when building on my personal Fedora 20 box.  But on a departmental RHEL 6 box with locally-built LLVM 3.3 and Boost 1.55.0, we run into problems without this tweak.
Prefer a slightly more modern GCC, if available.
Make a single-argument construction look more like initialization.

I generally prefer "=" initialization when there's only one argument.  For example:

````
int x = 5;
point p(5, 6);
````
Remove an unused construction variable.
Factor out common logic of two similar flag tweaks
On CS machines, look for our own local installation of LLVM.

This local installation was built using the CSL-installed gcc-4.9.0.  All of its executables and shared libraries have the proper RPATH settings to find the correct gcc-4.9.0 C++ runtime library.  That, in turn, means we can also use gcc-4.9.0 for building our own LLVM passes here.