Link with -Bsymbolic to make sure we use the local copy of houdini

If `escape_utils` is used with another gem (like `redcarpet`) that also has a copy of houdini inside then the dynamic linker can wind up using the wrong version of the houdini routines.

Adding `-Bsymbolic` to the linker flags tells the linker to prefer the local version when resolving symbols.

I assume this will actually need some guarding so that it only happens on ELF systems using the `binutils` linker but I have no idea how to achieve that...