[devel] Freeze build-time only configuration in the build.

    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

scripts/symbols.def

diff --git a/scripts/symbols.def b/scripts/symbols.def
index f650f72..b2b445e 100644
--- a/scripts/symbols.def
+++ b/scripts/symbols.def
@@ -1,5 +1,5 @@
 ;--------------------------------------------------------------
-; LIBPNG symbol list as a WIn32 DEF file
+; LIBPNG symbol list as a Win32 DEF file
 ; Contains all the symbols that can be exported from libpng
 ;--------------------------------------------------------------
 LIBRARY
@@ -54,7 +54,7 @@
  png_set_invert_mono @46
  png_set_background @47
  png_set_strip_16 @48
-; png_set_dither @49
+ png_set_quantize @49
  png_set_gamma @50
  png_set_flush @51
  png_write_flush @52
@@ -213,4 +213,3 @@
  png_save_uint_32 @205
  png_save_int_32 @206
  png_save_uint_16 @207
- png_set_quantize @208