Fix compile errors on Illumos

Tested in a SmartOS zone.

Needed to include limits.h for this:
```
building src/libstore/build.o
  CXX    src/libstore/build.o
src/libstore/build.cc: In member function 'void nix::Worker::waitForInput()':
src/libstore/build.cc:3153:22: error: 'LONG_MAX' was not declared in this scope
     time_t nearest = LONG_MAX; // nearest deadline
                      ^
```

And -lsocket for missing symbols:
```
Undefined                       first referenced
 symbol                             in file
__xnet_socket                       src/nix-daemon/nix-daemon.o  (symbol belongs to implicit dependency /lib/amd64/libsocket.so.1)
accept                              src/nix-daemon/nix-daemon.o  (symbol belongs to implicit dependency /lib/amd64/libsocket.so.1)
listen                              src/nix-daemon/nix-daemon.o  (symbol belongs to implicit dependency /lib/amd64/libsocket.so.1)
__xnet_bind                         src/nix-daemon/nix-daemon.o  (symbol belongs to implicit dependency /lib/amd64/libsocket.so.1)

ld: fatal: symbol referencing errors. No output written to src/nix-daemon/nix-daemon
collect2: error: ld returned 1 exit status
```