build: set HAVE_WRITABLE_ARGV for Apple cross-builds

Enable this feature for Apple cross-builds to match native macOS builds.

Closes #16338

CMakeLists.txt

@@ -546,6 +546,7 @@ if(_CURL_QUICK_DETECT)
   elseif(APPLE)
     set(HAVE_EVENTFD 0)
     set(HAVE_GETPASS_R 0)
+    set(HAVE_WRITABLE_ARGV 1)
     set(HAVE_SENDMMSG 0)
   endif()
 endif()

configure.ac

@@ -591,13 +591,16 @@ dnl **********************************************************************
 CURL_CHECK_WIN32_LARGEFILE
 CURL_CHECK_WIN32_CRYPTO
 
+curl_cv_apple='no'
+case $host in
+  *-apple-*) curl_cv_apple='yes';;
+esac
+
 CURL_DARWIN_CFLAGS
 
-case $host in
-  *-apple-*)
-    CURL_SUPPORTS_BUILTIN_AVAILABLE
-    ;;
-esac
+if test "$curl_cv_apple" = 'yes'; then
+  CURL_SUPPORTS_BUILTIN_AVAILABLE
+fi
 
 curl_cv_cygwin='no'
 case $host_os in
@@ -1788,6 +1791,9 @@ int main(int argc, char **argv)
 ],[
   curl_cv_writable_argv=cross
 ])
+if test "$curl_cv_writable_argv" = 'cross' -a "$curl_cv_apple" = 'yes'; then
+  curl_cv_writable_argv=yes
+fi
 case $curl_cv_writable_argv in
   yes)
     AC_DEFINE(HAVE_WRITABLE_ARGV, 1, [Define this symbol if your OS supports changing the contents of argv])