Tell CMake to not check for a C++ compiler (#653)

By default CMake checks both for C and C++ compilers, while the latter
is not needed. Setting the list of languages to just "C" in the call to
project() removes the unneeded check.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2dc7232..99b9258 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -4,7 +4,7 @@
 # support 2.8.7.
 cmake_minimum_required(VERSION 2.8.6)
 
-project(brotli)
+project(brotli C)
 
 # If Brotli is being bundled in another project, we don't want to
 # install anything.  However, we want to let people override this, so