CVS domivogt: * Fixed png detection for cross compilation.

[FVWM CVS]

CVSROOT:        /home/cvs/fvwm
Module name:    fvwm
Changes by:     domivogt        08/08/08 06:50:16

Modified files:
        .              : ChangeLog configure.ac 

Log message:
* Fixed png detection for cross compilation.