util/llvmdisassembler: build with ac++-clang

This change updates the documentation on how to manually build LLVM
3.3 or 3.4, and removes the forced -fno-rtti compiler flag from
util/llvmdisassembler/: The flag breaks compilation with (not yet
released) AspectC++ 2.0, which complains about dynamic_cast (used in
some active aspect headers that should not affect llvmdisassembler at
all) being unusable with -fno-rtti.  It's probably not a good idea to
compile only a part of Fail* with -fno-rtti anyways.

Note that the Debian and Ubuntu LLVM packages are built with
REQUIRES_RTTI=1, too.

Change-Id: I9891cf074d4201df786f7f5a9b96033e18832562

doc/how-to-build.txt

@@ -19,9 +19,13 @@ Required for Fail*:
    <http://akut.aspectc.org>.  Make sure you use the 64-bit version if running
    in a 64-bit environment.
  - optional:
-   * LLVM 3.3 (needed for several importers in tools/import-trace)
+   * LLVM 3.3 or 3.4 (needed for several importers in tools/import-trace)
      (compiles/links with 3.1 or 3.2, but fails to properly import information
      from ELF binaries not compiled with -ffunction-sections)
+     -  configured with, e.g., --enable-optimized --disable-assertions
+        --disable-werror
+     -  built with "make REQUIRES_RTTI=1" (the Debian/Ubuntu packages already
+        come built this way)
    * a MySQL 5.0+ or MariaDB 5.1+ (MariaDB 5.5 recommended) server