In all prior versions of libpng most configuration options
controlled by compiler #defines had to be repeated by the
application code that used libpng. This patch changes this
so that compilation options that can only be changed at build
time are frozen in the build. Options that are compiler
dependent (and those that are system dependent) are evaluated
each time - pngconf.h holds these. Options that can be changed
per-file in the application are in png.h. Frozen options are
in the new installed header file pnglconf.h
and callbacks.
Changed rules for generation of the various symbol files and added a new
rule for a DEF file (which is also added to the distribution).
Updated the symbol file generation to stop it adding spurious spaces
to EOL (coming from preprocessor macro expansion). Added a facility
to join tokens in the output and rewrite *.dfn to use this.
