curl: Do not use libnetwork on Apple platforms

Starting with OS X 10.11 there is a library called libnetwork
which will be picked up during curl configuration.
This breaks backward compatibility of the resulting binaries
because libnetwork is not available on older OS X versions.

Utilities/cmcurl/CMakeLists.txt

@@ -43,6 +43,14 @@ if(WIN32)
   set(HAVE_INET_PTON 0 CACHE INTERNAL "Do not use inet_pton")
 endif()
 
+# Starting with OSX 10.11 there is an unrelated libnetwork library which will
+# be picked up during curl configuration. Linking against this library is
+# unnecessary and breaks backward compatibility of the resulting binaries
+# because libnetwork is unavailable on older OSX versions.
+if(APPLE)
+  set(HAVE_LIBNETWORK 0 CACHE INTERNAL "Do not use libnetwork")
+endif(APPLE)
+
 # Disable warnings to avoid changing 3rd party code.
 if(CMAKE_C_COMPILER_ID MATCHES
     "^(GNU|Clang|AppleClang|XL|VisualAge|SunPro|MIPSpro|HP|Intel)$")