Build: Make Lagom build under macOS (#2341)

Lagom now builds under macOS. Only two minor adjustments were required:

* LibCore TCP/UDP code can't use `SOCK_{NONBLOCK,CLOEXEC}` on macOS,
use ioctl() and fcntl() instead

* LibJS `Heap` code pthread usage ported to MacOS

Libraries/LibCore/UDPSocket.cpp

@@ -28,6 +28,10 @@
 #include <errno.h>
 #include <sys/socket.h>
 
+#ifndef SOCK_NONBLOCK
+#    include <sys/ioctl.h>
+#endif
+
 namespace Core {
 
 UDPSocket::UDPSocket(int fd, Object* parent)
@@ -43,7 +47,14 @@ UDPSocket::UDPSocket(int fd, Object* parent)
 UDPSocket::UDPSocket(Object* parent)
     : Socket(Socket::Type::UDP, parent)
 {
+#ifdef SOCK_NONBLOCK
     int fd = socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0);
+#else
+    int fd = socket(AF_INET, SOCK_DGRAM, 0);
+    int option = 1;
+    ioctl(fd, FIONBIO, &option);
+#endif
+
     if (fd < 0) {
         set_error(errno);
     } else {