Revert "Makefile: Don't force any C++ dialect, instead use the host compiler's default.", because we can't have nice things (failed on the mobile branch and blocking promotions).

This reverts commit de845f9.

Author: brycelelbach

Makefile

@@ -30,8 +30,8 @@
 
 # Makefile for building Thrust unit test driver
 
-# Don't force any C++ mode, use the host compiler's default.
-export CXX_STD =
+# Force C++11 mode. NVCC will ignore it if the host compiler doesn't support it.
+#export CXX_STD = c++11
 
 export VERBOSE = 1