cmake: stop including files from the install directory

Summary:
Here is the buggy behavior which this change fixes:

* On the first configure with CMake, a system-wide benchmark installation is not found, so we use the version in `third_party/` ([see here](https://github.com/caffe2/caffe2/blob/v0.8.1/cmake/Dependencies.cmake#L98-L100))
* On installation, the benchmark sub-project installs its headers to `CMAKE_INSTALL_PREFIX` ([see here](https://github.com/google/benchmark/blob/4bf28e611b/src/CMakeLists.txt#L41-L44))
* On a rebuild, CMake searches the system again for a benchmark installation (see facebookarchive/caffe2#916 for details on why the first search is not cached)
* CMake includes `CMAKE_INSTALL_PREFIX` when searching the system ([docs](https://cmake.org/cmake/help/v3.0/variable/CMAKE_SYSTEM_PREFIX_PATH.html))
* Voila, a "system" installation of benchmark is found at `CMAKE_INSTALL_PREFIX`
* On a rebuild, `-isystem $CMAKE_INSTALL_PREFIX/include` is added to every build target ([see here](https://github.com/caffe2/caffe2/blob/v0.8.1/cmake/Dependencies.cmake#L97)). e.g:

      cd /caffe2/build/caffe2/binaries && ccache /usr/bin/c++    -I/caffe2/build -isystem /caffe2/third_party/googletest/googletest/include -isystem /caffe2/install/include -isystem /usr/include/opencv -isystem /caffe2/third_party/eigen -isystem /usr/include/python2.7 -isystem /usr/lib/python2.7/dist-packages/numpy/core/include -isystem /caffe2/third_party/pybind11/include -isystem /usr/local/cuda/include -isystem /caffe2/third_party/cub -I/caffe2 -I/caffe2/build_host_protoc/include  -fopenmp -std=c++11 -O2 -fPIC -Wno-narrowing -O3 -DNDEBUG   -o CMakeFiles/split_db.dir/split_db.cc.o -c /caffe2/caffe2/binaries/split_db.cc

This causes two issues:
1. Since the headers and libraries at `CMAKE_INSTALL_PREFIX` have a later timestamp than the built files, an unnecessary rebuild is triggered
2. Out-dated headers from the install directory are used during compilation, which can lead to strange build errors (which can usually be fixed by `rm -rf`'ing the install directory)

Possible solutions:
* Stop searching the system for an install of benchmark, and always use the version in `third_party/`
* Cache the initial result of the system-wide search for benchmark, so we don't accidentally pick up the installed version later
* Hack CMake to stop looking for headers and libraries in the installation directory

This PR is an implementation of the first solution. Feel free to close this and fix the issue in another way if you like.
Closes facebookarchive/caffe2#1112

Differential Revision: D5761750

Pulled By: Yangqing

fbshipit-source-id: 2240088994ffafdb6eedb3626d898b505a4ba564

cmake/Modules/FindBenchmark.cmake

@@ -4,22 +4,24 @@
 #  Benchmark_LIBRARIES    - libraries needed to use benchmark
 
 find_path(Benchmark_INCLUDE_DIR
-	NAMES benchmark/benchmark.h
-	DOC "The directory where benchmark includes reside"
+  NAMES benchmark/benchmark.h
+  NO_SYSTEM_ENVIRONMENT_PATH
+  DOC "The directory where benchmark includes reside"
 )
 
 find_library(Benchmark_LIBRARY
-	NAMES benchmark
-	DOC "The benchmark library"
+  NAMES benchmark
+  NO_SYSTEM_ENVIRONMENT_PATH
+  DOC "The benchmark library"
 )
 
 set(Benchmark_INCLUDE_DIRS ${Benchmark_INCLUDE_DIR})
 set(Benchmark_LIBRARIES    ${Benchmark_LIBRARY})
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(Benchmark
-	FOUND_VAR Benchmark_FOUND
-	REQUIRED_VARS Benchmark_INCLUDE_DIR Benchmark_LIBRARY
+  FOUND_VAR Benchmark_FOUND
+  REQUIRED_VARS Benchmark_INCLUDE_DIR Benchmark_LIBRARY
 )
 
 mark_as_advanced(Benchmark_FOUND)