build: Rename a private function to benefit Embarcadero C++Builder

Embarcadero's compilers, old (Borland-based) and new (Clang-based),
do have full support for Standard C. The Clang-based compiler is
claiming, through the macro __STDC_VERSION__, to support C99 and C11,
whereas the Borland-based compiler supports ANSI C, as it has for
decades.

However, their run-time library is exposing global functions beyond
the scope of Standard C, for backwards compatibility with the older
Borland products. One of these functions is `randomize`, which clashes
with a function inside pngvalid.c that incidentally has the same name.

Building libpng in "Strict ANSI C" mode, in which all Borland-specific
globals are hidden (e.g. via `bcc32 -A`), would have been a workable
solution for the Borland-based ANSI C compiler, but no such solution
appears to exist for the Clang-based C90/C99/C11 compiler.

Fortunately, renaming a private function inside a test program is a
cost-free alternative fix.

contrib/libtests/pngvalid.c

@@ -303,20 +303,20 @@ make_four_random_bytes(png_uint_32* seed, png_bytep bytes)
 #if defined PNG_READ_SUPPORTED || defined PNG_WRITE_tRNS_SUPPORTED ||\
     defined PNG_WRITE_FILTER_SUPPORTED
 static void
-randomize(void *pv, size_t size)
+randomize_bytes(void *pv, size_t size)
 {
    static png_uint_32 random_seed[2] = {0x56789abc, 0xd};
    make_random_bytes(random_seed, pv, size);
 }
 
-#define R8(this) randomize(&(this), sizeof (this))
+#define R8(this) randomize_bytes(&(this), sizeof (this))
 
 #ifdef PNG_READ_SUPPORTED
 static png_byte
 random_byte(void)
 {
    unsigned char b1[1];
-   randomize(b1, sizeof b1);
+   randomize_bytes(b1, sizeof b1);
    return b1[0];
 }
 #endif /* READ */
@@ -325,7 +325,7 @@ static png_uint_16
 random_u16(void)
 {
    unsigned char b2[2];
-   randomize(b2, sizeof b2);
+   randomize_bytes(b2, sizeof b2);
    return png_get_uint_16(b2);
 }
 
@@ -335,7 +335,7 @@ static png_uint_32
 random_u32(void)
 {
    unsigned char b4[4];
-   randomize(b4, sizeof b4);
+   randomize_bytes(b4, sizeof b4);
    return png_get_uint_32(b4);
 }
 #endif /* READ_FILLER || READ_RGB_TO_GRAY */