build: remove forced -fno-rtti

build: remove forced -fno-rtti

* As discussed on the mailing list,
  forced no-rtti breaks C++ public
  API's such as the Haiku C++ libGL.so
* -fno-rtti *can* be still set however
  instead of blindly forcing -fno-rtti,
  we can rely on the llvm-config
  --cppflags output.
  If the system llvm is built without
  rtti (default), the no-rtti flag will be
  present in llvm-config --cppflags
  (which we pick up on)
  If llvm is built with rtti
  (REQUIRES_RTTI=1), then -fno-rtti is
  removed from llvm-config --cppflags.
* We could selectively add / remove rtti
  from various components, however mixing
  rtti and non-rtti code is tricky and
  could introduce missing symbols.
* This needs impact tested.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>