Revert "Not using libc++ on FreeBSD."

This reverts commit b451674f88e4e83c904379f64b1bb3438413c1e4.

cpp/CMakeLists.txt

@@ -58,8 +58,7 @@ if ( CMAKE_CXX_COMPILER_ID STREQUAL "Clang" )
   set( COMPILER_IS_CLANG true )
 
   # The Travis CI build machines don't have libc++ installed
-  # Also, FreeBSD complains about this too. See issue #349.
-  if ( NOT DEFINED ENV{TRAVIS} AND NOT SYSTEM_IS_FREEBSD )
+  if ( NOT DEFINED ENV{TRAVIS} )
     set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++" )
   endif()
 endif()