[libpng17] Fixed Windows build issues, enabled ARM compilation. Various warnings issued

by earlier versions of GCC fixed for Cygwin and Min/GW (which both use old GCCs.)
ARM support is enabled by default in zlib.props (unsupported by Microsoft) and
ARM compilation is made possible by deleting the check for x86. The test programs
cannot be run because they are not signed.

pngstruct.h

@@ -148,8 +148,16 @@ struct png_struct_def
     * (char), (short), (int) and pointer types are kept separate, however
     * associated members under the control of the same #define are still
     * together.
-    *
-    * First the frequently accessed fields.  Many processors perform arithmetic
+    */
+#ifdef PNG_SETJMP_SUPPORTED
+   /* jmp_buf can have very high alignment requirements on some systems, so put
+    * it first (the other setjmp members are later as they are infrequently
+    * accesed.)
+    */
+   jmp_buf jmp_buf_local;
+#endif
+
+   /* Next the frequently accessed fields.  Many processors perform arithmetic
     * in the address pipeline, but frequently the amount of addition or
     * subtraction is limited.  By putting these fields at the head of png_struct
     * the hope is that such processors will generate code that is both smaller
@@ -198,10 +206,6 @@ struct png_struct_def
 
    /* ERROR HANDLING */
 #ifdef PNG_SETJMP_SUPPORTED
-   /* jmp_buf can have very high alignment requirements on some systems, so put
-    * it first.
-    */
-   jmp_buf         jmp_buf_local;
    jmp_buf        *jmp_buf_ptr;   /* passed to longjmp_fn */
    png_longjmp_ptr longjmp_fn;    /* setjmp non-local goto function. */
    size_t          jmp_buf_size;  /* size of *jmp_buf_ptr, if allocated */