depends: Fix "unused variables" warning when compiling zeromq for Windows

Clang triggers that warning, which becomes an error due to -Werror.

depends/packages/zeromq.mk

@@ -3,6 +3,7 @@ $(package)_version=4.3.3
 $(package)_download_path=https://github.com/zeromq/libzmq/releases/download/v$($(package)_version)/
 $(package)_file_name=$(package)-$($(package)_version).tar.gz
 $(package)_sha256_hash=9d9285db37ae942ed0780c016da87060497877af45094ff9e1a1ca736e3875a2
+$(package)_patches=windows-unused-variables.diff
 
 ifneq ($(host_os),darwin)
 $(package)_dependencies=libcxx
@@ -19,6 +20,10 @@ define $(package)_set_vars
   $(package)_ldflags+=-static-libstdc++ -lc++abi
 endef
 
+define $(package)_preprocess_cmds
+  patch -p1 < $($(package)_patch_dir)/windows-unused-variables.diff
+endef
+
 define $(package)_config_cmds
   $($(package)_autoconf)
 endef

depends/patches/zeromq/windows-unused-variables.diff

@@ -0,0 +1,17 @@
+diff -ur zeromq-4.3.3-orig/src/clock.cpp zeromq-4.3.3/src/clock.cpp
+--- zeromq-4.3.3-orig/src/clock.cpp	2020-10-13 13:44:04.190000000 +0100
++++ zeromq-4.3.3/src/clock.cpp	2020-10-13 13:47:04.170000000 +0100
+@@ -126,9 +126,13 @@
+   init_compatible_get_tick_count64 ();
+ #endif
+ 
++#ifndef ZMQ_HAVE_WINDOWS
+ const uint64_t usecs_per_msec = 1000;
++#endif
+ const uint64_t usecs_per_sec = 1000000;
++#ifndef ZMQ_HAVE_WINDOWS
+ const uint64_t nsecs_per_usec = 1000;
++#endif
+ 
+ zmq::clock_t::clock_t () :
+     _last_tsc (rdtsc ()),