Mark CMake project as C++-only (#681)

This will make CMake skip all the checks for C compiler.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 74b9934..41b3b66 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,7 +12,7 @@
   endif()
 endforeach()
 
-project (benchmark)
+project (benchmark CXX)
 
 option(BENCHMARK_ENABLE_TESTING "Enable testing of the benchmark library." ON)
 option(BENCHMARK_ENABLE_EXCEPTIONS "Enable the use of exceptions in the benchmark library." ON)