Updated Linux binaries to work with different libpng versions (helps #89).

The compiled binary always has version of libpng as 1.2.0 (even though
wxwidgets may come with a different version) and is compiled with libpng
set to use system library. The warnings from libpng about different
versions are suppressed when bitmaps are loaded. This has been tested to
work with libpng 1.2.x and 1.5.x.

This update also adds a workaround for "lsb_release: command not found"
message reported on some Linux systems.