Symbol prefixing: protection against CC/CPPFLAGS/CFLAGS useage
Previous iOS/Xcode fixes for the ARM NEON optimizations moved the test on
__ARM_NEON__ from configure time to compile time.  This breaks symbol prefixing
because the definition of the special png_init_filter_functions call was hidden
at configure time if the relevant compiler arguments are passed in CFLAGS as
opposed to CC.  This change attempts to avoid all the confusion that would
result by declaring the init function even when it is not used, so that it will
always get prefixed.

Signed-off-by: John Bowler <jbowler@acm.org>