Try to detect if --with-readline-cflags is passed a non-cflags value, e.g. readline...

Try to detect if --with-readline-cflags is passed a non-cflags value, e.g. readline/readline.h, and fail loudly if it is. The legacy configure accepted that formulation but silently did nothing useful with it and that formulation now causes the auto-completion test to fail, as reported in [forum:f13b8e8f38402367 | forum post f13b8e8f38402367].

FossilOrigin-Name: c97f34d064c1fc1e6860761a417aaab2a79bd3871bcd1dad1d0d3b6f4b692a1e