Use CXXFLAGS_CXX11 when detecting Clang plugin headers

...which is necessary for recent Clang trunk towards 3.5, which is implemented
in C++11 now, and should not hurt for older versions.  Required to move the
plugin check after the check that determines CXXFLAGS_CXX11.

Change-Id: I73aff79e09749e9d7c6ea813422f35c1b8475041