Defensive checks for vmnet.framework

Require targeting macOS 10.10, in case the user builds with an older SDK
or sets the target version to an older release. This shouldn't be a
problem with modern macOS SDKs, but users using or targeting older macOS
should have a better message available rather than a compile message.
The check is ugly, but it should work.

A similar check is done for block support; this should only be a problem
with users using GCC instead of clang, which is default on macOS.

Also add a message for vmnet support when printing the build config.

cmake/dep-link.cmake

@@ -352,8 +352,33 @@ if (WITH_NETWORK)
     endif (WITH_SLIRP)
 
     if (WITH_VMNET AND APPLE)
+        # CMAKE_OSX_DEPLOYMENT_TARGET is attractive, but not set by default.
+        # See what we're using, either by default or what the user has set.
+        check_c_source_compiles(
+            "
+            #include <Availability.h>
+            #if TARGET_OS_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED < 101000
+            #error macOS too old
+            #endif
+            int main(int argc, char **argv) { return 0; }
+            " TARGETING_MACOS_10_10)
+        if (NOT TARGETING_MACOS_10_10)
+        message(FATAL_ERROR "vmnet.framework requires targeting macOS 10.10 or newer")
+        endif()
+
+        # vmnet requires blocks for its callback parameter, even in vanilla C.
+        # This is only supported in clang, not by GCC. It's default in clang,
+        # but we should be clear about it. Do a feature instead of compiler
+        # check anyways though, in case GCC does add it eventually.
+        check_c_compiler_flag(-fblocks HAVE_C_BLOCKS)
+        if (NOT HAVE_C_BLOCKS)
+            message(FATAL_ERROR "vmnet.framework requires blocks support in the C compiler")
+        endif()
+        target_compile_options(simh_network INTERFACE -fblocks)
+
         target_link_libraries(simh_network INTERFACE "-framework vmnet")
         target_compile_definitions(simh_network INTERFACE HAVE_VMNET_NETWORK)
+        list(APPEND NETWORK_PKG_STATUS "macOS vmnet.framework")
     endif(WITH_VMNET AND APPLE)
 
     ## Finally, set the network runtime