cmake: Work around broken sys/random.h in old macOS SDKs

Fixes #820.

Author: nwellnhof

CMakeLists.txt

@@ -149,7 +149,12 @@ if (NOT MSVC)
     check_function_exists(stat HAVE_STAT)
     check_include_files(stdint.h HAVE_STDINT_H)
     check_include_files(sys/mman.h HAVE_SYS_MMAN_H)
-    check_include_files(sys/random.h HAVE_SYS_RANDOM_H)
+    if (APPLE)
+        # In old macOS SDKs (ex: 10.15), sys/random.h fails to include header files it needs, so add them here.
+        check_include_files("Availability.h;stddef.h;sys/random.h" HAVE_SYS_RANDOM_H)
+    else()
+        check_include_files(sys/random.h HAVE_SYS_RANDOM_H)
+    endif()
     check_include_files(sys/select.h HAVE_SYS_SELECT_H)
     check_include_files(sys/socket.h HAVE_SYS_SOCKET_H)
     check_include_files(sys/stat.h HAVE_SYS_STAT_H)