The check itself was always failing unless you added 0x to cxxflags
and we can safely assume the header to be there.
If the header fails during compile time it throws nice
error explaining what is going on anyway.

Change-Id: Iedff10f3b2562f731063107df7220efa124aca08
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>