configure: Keep LIBPNG_CFLAGS settings if png.h is available.

[Piotr Caban]

It fixes #ifdef HAVE_PNG_H checks if library is not available.

Signed-off-by: Piotr Caban 
---
  configure.ac | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

[Alexandre Julliard]

Piotr Caban  writes:
> It fixes #ifdef HAVE_PNG_H checks if library is not available.

It would be better to fix the code to check SONAME_LIBPNG, there's no
reason to include the headers in that case.