cmake test: skip case where Cmake backend not Fortran-compatible

test cases/cmake/14 fortran threads/meson.build

@@ -6,4 +6,7 @@ endif
 
 # want to be sure that CMake can find dependencies where even if the
 # project isn't C, the C language is required to find the library.
-threads = dependency('threads', method: 'cmake', required: true)
+threads = dependency('threads', method: 'cmake', required: false)
+if not threads.found()
+  error('MESON_SKIP_TEST: CMake backend not working for Fortran / threads')
+endif