configure.ac: Fix breakage caused by -Werror=unused-variable

[Daniel Santos]

On 09/07/2015 04:53 AM, Alexandre Julliard wrote:
> Daniel Santos <daniel.santos at pobox.com> writes:
>
>> When configuring with CFLAGS="-Wunused-variable -Werror", the configure
>> test for HAVE_PNG_SET_EXPAND_GRAY_1_2_4_TO_8 fails with unused variable.
> Running configure with -Werror is not a good idea. Is there a particular
> reason why you need this?
>
Not for configure in particular, but for the build I prefer to have the 
highest level of warnings enabled (that won't produce spam) and I've 
found -Werror a good way to make sure that none slip past me.

Daniel