configure.ac:	Add test for c++ compiler.

The AC_PROG_CXX macro sets a flag if a C++ compiler is found
but does not fail if one is not found, it left to application
to test the flags as needed. This fix will issue an error
when a c++ compiler is not found.  Bug 3955.

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>

it's not installed, this fix adds an error message for a
Merge git://github.com/ceph/ceph