[libpng17] Corrected Android builds and corrected libpng.vers with symbol

prefixing. This adds an API to set optimization options externally,
providing an alternative and general solution for the non-portable
run-time tests used by the ARM Neon code.  It also makes those tests
compile and link on Android.  The order of settings vs options in
pnglibconf.h is reversed to allow settings to depend on options and
options can now set (or override) the defaults for settings.

scripts/checksym.awk

@@ -113,6 +113,16 @@ END{
       err = 1
    }
    unexported=0
+   # Add a standard header to symbols.new:
+   print ";Version INSERT-VERSION-HERE" >of
+   print ";--------------------------------------------------------------" >of
+   print "; LIBPNG symbol list as a Win32 DEF file" >of
+   print "; Contains all the symbols that can be exported from libpng" >of
+   print ";--------------------------------------------------------------" >of
+   print "LIBRARY" >of
+   print "" >of
+   print "EXPORTS" >of
+
    for (o=1; o<=lasto; ++o) {
       if (symbol[o] == "" && removed[o] == "") {
          if (unexported == 0) unexported = o