expat asks for entropy the graph has, instead of one no musl provides (#451)

`lib/expat_config.h` is one static header shared by every target, and it
carried `#define HAVE_ARC4RANDOM_BUF 1` from a glibc configure run. Over
openkal-musl the member stopped at

    xmlparse.c:977:3: error: call to undeclared function 'arc4random_buf'

musl 1.2.5 has no arc4random at all --- its `src/prng/` is the rand48 family,
and the name appears in no header. This is therefore a recipe that claims a
function for hosts that do not have it, not a gap in the C library.

THE MACRO SHORT-CIRCUITS THE WHOLE CHAIN, which is why the fallbacks below it
never ran. `generate_hash_secret_salt` reads:

    #if defined(HAVE_ARC4RANDOM_BUF)
      arc4random_buf(&entropy, sizeof(entropy));
    #elif defined(HAVE_ARC4RANDOM)
    #else
      /* Try high quality providers first .. */
    #  elif defined(HAVE_GETRANDOM) || defined(HAVE_SYSCALL_GETRANDOM)

Undefined, the chain falls to `HAVE_GETRANDOM`, already defined in this same
header and provided by both musl and glibc, and then to `XML_DEV_URANDOM`. No
target loses a high-quality entropy source; the glibc hosts that were reaching
arc4random_buf reach getrandom instead.

Verified against the published stack --- openkal-linux 0.15.0, openkal-musl
0.18.0, openkal-llvm-runtime 0.13.0 --- with this checkout as a live index:
expat 2.7.1 compiles and the program links and runs, and the artefact
references no arc4random symbol.

Author: Sunrisepeak

pkgs/c/compat.expat.lua

@@ -134,7 +134,17 @@ package = {
 /* #undef HAVE_ARC4RANDOM */
 
 /* Define to 1 if you have the `arc4random_buf' function. */
-#define HAVE_ARC4RANDOM_BUF 1
+/* NOT DEFINED, AND THIS HEADER IS SHARED BY EVERY TARGET. The value was
+   carried over from a glibc configure run. expat's selection is a chain that
+   `HAVE_ARC4RANDOM_BUF` SHORT-CIRCUITS -- `generate_hash_secret_salt` calls
+   `arc4random_buf` unconditionally under it and never reaches the getrandom
+   or /dev/urandom branches below. musl 1.2.5 has no arc4random at all (its
+   `src/prng/` is the rand48 family), so over openkal-musl the member stopped
+   at `call to undeclared function 'arc4random_buf'`.
+   Undefined, the chain falls to HAVE_GETRANDOM, which is defined below and
+   which musl and glibc both provide, and then to XML_DEV_URANDOM. No target
+   loses a high-quality entropy source. */
+/* #undef HAVE_ARC4RANDOM_BUF */
 
 /* define if the compiler supports basic C++11 syntax */
 /* #undef HAVE_CXX11 */