BUILD: xxhash: silence a build warning on Solaris + gcc-5.5

BUILD: xxhash: silence a build warning on Solaris + gcc-5.5

Testing an undefined macro emits warnings due to -Wundef, and we have
exactly one such case in xxhash:

  include/import/xxhash.h:3390:42: warning: "__cplusplus" is not defined [-Wundef]
   #if ((defined(sun) || defined(__sun)) && __cplusplus) /* Solaris includes __STDC_VERSION__ with C++. Tested with GCC 5.5 */

Let's just prepend "defined(__cplusplus) &&" before __cplusplus to
resolve the problem. Upstream is still affected apparently.