cannot find ffi.h

[dotnwat]

The README sounds like LLVM is optional, but when I build without LLVM-dev installed I get the following problem (installing LLVM fixes it).

nwatkins@kyoto:~/src/hornet/build$ cmake ..
-- The C compiler identification is GNU 4.9.2
-- The CXX compiler identification is GNU 4.9.2
-- Check for working C compiler: /usr/lib/ccache/cc
-- Check for working C compiler: /usr/lib/ccache/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/lib/ccache/c++
-- Check for working CXX compiler: /usr/lib/ccache/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Found JNI: /usr/lib/jvm/default-java/jre/lib/amd64/libjawt.so  
CMake Warning at cmake_modules/FindLLVM.cmake:126 (message):
  Could not find llvm-config.  Try manually setting LLVM_CONFIG to the
  llvm-config executable of the installation to use.
Call Stack (most recent call first):
  CMakeLists.txt:14 (find_package)


-- Could NOT find LLVM (missing:  LLVM_ROOT_DIR LLVM_HOST_TARGET) 
-- Configuring done
-- Generating done
-- Build files have been written to: /home/nwatkins/src/hornet/build
nwatkins@kyoto:~/src/hornet/build$ make
Scanning dependencies of target jvm
[  5%] Building CXX object CMakeFiles/jvm.dir/vm/alloc.cc.o
[ 10%] Building CXX object CMakeFiles/jvm.dir/vm/jvm.cc.o
[ 15%] Building CXX object CMakeFiles/jvm.dir/vm/klass.cc.o
[ 20%] Building CXX object CMakeFiles/jvm.dir/vm/object.cc.o
[ 25%] Building CXX object CMakeFiles/jvm.dir/vm/thread.cc.o
[ 30%] Building C object CMakeFiles/jvm.dir/mps/mps.c.o
[ 35%] Building CXX object CMakeFiles/jvm.dir/java/backend.cc.o
[ 40%] Building CXX object CMakeFiles/jvm.dir/java/class_file.cc.o
[ 45%] Building CXX object CMakeFiles/jvm.dir/java/constant_pool.cc.o
[ 50%] Building CXX object CMakeFiles/jvm.dir/java/ffi.cc.o
In file included from /home/nwatkins/src/hornet/java/ffi.cc:1:0:
/home/nwatkins/src/hornet/include/hornet/ffi.hh:5:17: fatal error: ffi.h: No such file or directory
 #include <ffi.h>
                 ^
compilation terminated.
make[2]: *** [CMakeFiles/jvm.dir/java/ffi.cc.o] Error 1
make[1]: *** [CMakeFiles/jvm.dir/all] Error 2
make: *** [all] Error 2