random: getentropy on macOS does not need unistd.h

change duplicate sys/random.h to random.h in AC_MSG_CHECK
remove definitions that are applicable to macos which include unistd.h
additionally change comparator argument from NULL to nullptr when evaluating &getentropy

Inspired by: c13c97d

configure.ac

@@ -655,8 +655,8 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>]],
  [ AC_MSG_RESULT(no)]
 )
 
-AC_MSG_CHECKING(for getentropy via random.h)
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>
+AC_MSG_CHECKING([for getentropy via sys/random.h])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
  #include <sys/random.h>]],
  [[ getentropy(nullptr, 32) ]])],
  [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_GETENTROPY_RAND, 1,[Define this symbol if the BSD getentropy system call is available with sys/random.h]) ],

src/random.cpp

@@ -25,7 +25,7 @@
 #include <sys/syscall.h>
 #include <linux/random.h>
 #endif
-#if defined(HAVE_GETENTROPY) || (defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX))
+#if defined(HAVE_GETENTROPY)
 #include <unistd.h>
 #endif
 #if defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
@@ -169,7 +169,7 @@ void GetOSRand(unsigned char *ent32)
     }
 #elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
     // We need a fallback for OSX < 10.12
-    if (&getentropy != NULL) {
+    if (&getentropy != nullptr) {
         if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
             RandFailure();
         }