Fix unused platform check and configuration for macOS

In a similar manner as zlib (madler/zlib#895),
libpng contains a header configuration that's no longer valid and
hasn't been exercised for the macOS target.

- The target OS conditional macros are misused. Specifically
  `TARGET_OS_MAC` covers all Apple targets, including iOS, and it
  should not be checked with `#if defined` as they would always be
  defined (to either 1 or 0) on Apple platforms.
- `#include <fp.h>` no longer works for the macOS target and results
  in a compilation failure. macOS ships all required functions in
  `math.h`, and clients should use `math.h` instead.

This problem has not been noticed until a recent extension in clang
(llvm/llvm-project#74676) exposed the issue
and broke libpng builds on Apple platforms. The failure can be
reproduced now by adding `#include <TargetConditionals.h>` before the
block.

pngpriv.h

@@ -556,18 +556,8 @@
     */
 #  include <float.h>
 
-#  if (defined(__MWERKS__) && defined(macintosh)) || defined(applec) || \
-    defined(THINK_C) || defined(__SC__) || defined(TARGET_OS_MAC)
-   /* We need to check that <math.h> hasn't already been included earlier
-    * as it seems it doesn't agree with <fp.h>, yet we should really use
-    * <fp.h> if possible.
-    */
-#    if !defined(__MATH_H__) && !defined(__MATH_H) && !defined(__cmath__)
-#      include <fp.h>
-#    endif
-#  else
-#    include <math.h>
-#  endif
+#  include <math.h>
+
 #  if defined(_AMIGA) && defined(__SASC) && defined(_M68881)
    /* Amiga SAS/C: We must include builtin FPU functions when compiling using
     * MATH=68881