More aggressive check for getrandom() and getentropy() usability

In particular, they can be present, but not usable for a given
compatibility level. This is especially expected on iOS and Android.

configure.ac

@@ -794,8 +794,44 @@ dnl Checks for functions and headers
 
 AC_FUNC_ALLOCA
 AS_IF([test "x$EMSCRIPTEN" = "x"],[
-  AC_CHECK_FUNCS([arc4random arc4random_buf getrandom getentropy])
+  AC_CHECK_FUNCS([arc4random arc4random_buf])
   AC_CHECK_FUNCS([mmap mlock madvise mprotect])
+
+  AC_MSG_CHECKING(for getrandom with a standard API)
+  AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+#include <stdlib.h>
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+#ifdef HAVE_SYS_RANDOM_H
+# include <sys/random.h>
+#endif
+]], [[
+unsigned char buf;
+(void) getrandom((void *) &buf, 1U, 0U);
+  ]])],
+  [AC_MSG_RESULT(yes)
+   AC_CHECK_FUNCS([getrandom])],
+  [AC_MSG_RESULT(no)
+  ])
+
+  AC_MSG_CHECKING(for getentropy with a standard API)
+  AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+#include <stdlib.h>
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+#ifdef HAVE_SYS_RANDOM_H
+# include <sys/random.h>
+#endif
+]], [[
+unsigned char buf;
+(void) getentropy((void *) &buf, 1U);
+  ]])],
+  [AC_MSG_RESULT(yes)
+   AC_CHECK_FUNCS([getentropy])],
+  [AC_MSG_RESULT(no)
+  ])
 ])
 AC_CHECK_FUNCS([posix_memalign getpid nanosleep])
 AC_CHECK_FUNCS([memset_s explicit_bzero explicit_memset])

dist-build/ios.sh

@@ -20,6 +20,12 @@ export XCODEDIR=$(xcode-select -p)
 export IOS_SIMULATOR_VERSION_MIN=${IOS_SIMULATOR_VERSION_MIN-"6.0.0"}
 export IOS_VERSION_MIN=${IOS_VERSION_MIN-"6.0.0"}
 
+echo
+echo "Warnings related to headers being present but not usable are due to functions"
+echo "that didn't exist in the specified minimum iOS version level."
+echo "They can be safely ignored."
+echo
+
 mkdir -p $SIMULATOR32_PREFIX $SIMULATOR64_PREFIX $IOS32_PREFIX $IOS32s_PREFIX $IOS64_PREFIX || exit 1
 
 # Build for the simulator