fix -r19041 : if C++ compiler is not found, is not set

configure.ac

@@ -82,7 +82,7 @@ AC_CHECK_PROGS(PROG_STAT,gstat stat)
 AC_CHECK_PROGS(PROG_DATE,gdate date)
 AC_OPENMP
 
-if test x$ac_cv_env_CXX_set = x; then
+if test x$CXX = x; then
    AC_MSG_ERROR([No compiler C++ found])
 fi