AC_CHECK_FUNCS and -Werror

AC_CHECK_FUNC doesn't provide any information about the type of the
checked function and just uses "char".

In normal configuration processes, the test compilation produces a
warning, but succeeds, if the function is available with a different
type.

But if you have as CFLAG -Werror, for example if you use start the
configure process with
	CFLAGS+=-Werror ./configure
then warning abuot the different types will cause the test to fail
despite the function is available.

Possible solutions:
* Provide the possibility to specify the type of the function
* Provide the possibility to specify the header, where the function is
defined, when it is available.

Greetings
Peter Häring