cmake: NO_SYSTEM_ENVIRONMENT_PATH for benchmark

In addition to other changes made to benchmark, this is helpful because
find_*() will still find files under the install prefix if PREFIX/bin is
in your path.

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)