depends: Only use D_DARWIN_C_SOURCE when building miniupnpc on darwin

Author: fanquake

depends/packages/miniupnpc.mk

@@ -6,10 +6,9 @@ $(package)_sha256_hash=90dda8c7563ca6cd4a83e23b3c66dbbea89603a1675bfdb852897c2c9
 
 define $(package)_set_vars
 $(package)_build_opts=CC="$($(package)_cc)"
-$(package)_build_opts_darwin=OS=Darwin LIBTOOL="$($(package)_libtool)"
+$(package)_build_opts_darwin=LIBTOOL="$($(package)_libtool)"
 $(package)_build_opts_mingw32=-f Makefile.mingw
 $(package)_build_env+=CFLAGS="$($(package)_cflags) $($(package)_cppflags)" AR="$($(package)_ar)"
-$(package)_build_env+=CFLAGS=-D_DARWIN_C_SOURCE
 endef
 
 define $(package)_preprocess_cmds