How to set external library dir

[MengjiaoH]

Hi,

I am not sure if I can set external library dir. The following is what I got from --verbose. embree_DIR is a external library I used and I always set the DIR when I compile the project.
Here is the project's link: https://github.com/ospray/OSPRay
Thanks!

./config_gen.py ~/Desktop/ospray/ --preserve-environment --verbose
Running cmake in '/tmp/tmpGzRBwj'...
$ cmake /home/sci/mengjiao/Desktop/ospray
-- The C compiler identification is Clang 3.8.0
-- The CXX compiler identification is Clang 3.8.0
-- Check for working C compiler: /home/sci/mengjiao/.vim/bundle/YCM-Generator/fake-toolchain/Unix/clang
-- Check for working C compiler: /home/sci/mengjiao/.vim/bundle/YCM-Generator/fake-toolchain/Unix/clang -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /home/sci/mengjiao/.vim/bundle/YCM-Generator/fake-toolchain/Unix/clang++
-- Check for working CXX compiler: /home/sci/mengjiao/.vim/bundle/YCM-Generator/fake-toolchain/Unix/clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Git: /usr/bin/git (found version "2.13.6")
-- Found TBB: /opt/intel/tbb/include
CMake Error at cmake/ospray_macros.cmake:46 (message):
We did not find Embree installed on your system. OSPRay requires an Embree
installation >= v2.15.0, please download and extract Embree (or compile
Embree from source), then set the 'embree_DIR' variable to the installation
(or build) directory.
Call Stack (most recent call first):
cmake/configure_embree.cmake:20 (ospray_find_embree)
cmake/ospray_options.cmake:114 (INCLUDE)
CMakeLists.txt:50 (INCLUDE)

-- Configuring incomplete, errors occurred!
See also "/tmp/tmpGzRBwj/CMakeFiles/CMakeOutput.log".

Running make...
$ make -i -j40
make: *** No targets specified and no makefile found. Stop.

Cleaning up...

Build completed in 2.13 sec

Collected 0 relevant entries for C compilation (0 discarded).
Collected 0 relevant entries for C++ compilation (0 discarded).

ERROR: No commands were logged to the build logs (C: /tmp/tmpvfLOBv, C++: /tmp/tmppsmtoV).
Your build system may not be compatible.