have_header(header, preheaders = nil, opt = "", &b)
Instance Public methods
Returns whether or not the given header file can be found on
your system. If found, a macro is passed as a preprocessor constant to the
compiler using the header file name, in uppercase, prepended with
HAVE_.
For example, if have_header('foo.h') returned true,
then the HAVE_FOO_H preprocessor macro would be passed to the
compiler.