Set standard variables for package after probing

Standard variables must be set in order for the package to be picked
up with the OpmFind/OpmPackage dance.

cmake/Modules/FindCXX11Features.cmake

@@ -389,4 +389,6 @@ if(CXX_FEATURES_MISSING)
       "Your compiler does not seem to implement the following features:\n"
       "${CXX_FEATURES_MISSING}")
   endif()
+else ()
+  set (CXX11FEATURES_FOUND TRUE)
 endif()

cmake/Modules/FindCxaDemangle.cmake

@@ -15,3 +15,9 @@ int main(void){
     int status;
     char *demangled = abi::__cxa_demangle( foo, 0, 0, &status );
 }" HAVE_CXA_DEMANGLE)
+
+include (FindPackageHandleStandardArgs)
+# prevent useless message from being displayed
+set (FIND_PACKAGE_MESSAGE_DETAILS_CxaDemangle "[1][v()]"
+  CACHE INTERNAL "Details about finding CxaDemangle")
+find_package_handle_standard_args (CxaDemangle DEFAULT_MSG HAVE_CXA_DEMANGLE)