[libpng16] Fixed missing dependency in --prefix builds The intermediate

internal 'prefix.h' file can only be generated correctly after
pnglibconf.h, however the dependency was not in Makefile.am.  The
symptoms are unpredictable depending on the order make chooses to
build pngprefix.h and pnglibconf.h, often the error goes unnoticed
because there is a system pnglibconf.h to use instead.
3 files changed