explict cxx

cmake/external/libxsmm.cmake

@@ -41,7 +41,7 @@ ExternalProject_Add(
     UPDATE_COMMAND  ""
     CONFIGURE_COMMAND ""
     BUILD_IN_SOURCE 1
-    BUILD_COMMAND   $(MAKE) --silent PREFIX=${LIBXSMM_INSTALL_DIR} CC=gcc WARP=0 install
+    BUILD_COMMAND   $(MAKE) --silent PREFIX=${LIBXSMM_INSTALL_DIR} CXX=g++ CC=gcc WARP=0 install
     INSTALL_COMMAND ""
 )
 ADD_LIBRARY(libxsmm STATIC IMPORTED GLOBAL)