Fix detection of libffi on MacOS (or other OS with dodgy system libff…

…i). Based on patch from Jeremy Hinegardner <jeremy@hinegardner.org>

ext/ffi_c/extconf.rb

@@ -4,13 +4,16 @@
 dir_config("ffi_c")
 
 unless Config::CONFIG['host_os'] =~ /mswin32|mingw32/
-  pkg_config("libffi") || find_header("ffi.h", "/usr/local/include", "/opt/local/include")
-  have_ffi_call = have_library("ffi", "ffi_call", [ "ffi.h" ])
-  have_prep_closure = have_func("ffi_prep_closure")
-  libffi_ok = have_ffi_call && have_prep_closure
-  $defs << "-DHAVE_LIBFFI" if libffi_ok
-  $defs << "-DHAVE_RAW_API" if have_func("ffi_raw_call") && have_func("ffi_prep_raw_closure")
+  if pkg_config("libffi") || find_header("ffi.h", "/usr/local/include", "/opt/local/include")
+
+    # We need at least ffi_call and ffi_prep_closure
+    libffi_ok = have_library("ffi", "ffi_call", [ "ffi.h" ]) && have_func("ffi_prep_closure")
+
+    # Check if the raw api is available.
+    $defs << "-DHAVE_RAW_API" if have_func("ffi_raw_call") && have_func("ffi_prep_raw_closure")
+  end
 end
+
 have_func('rb_thread_blocking_region')
 
 $defs << "-DHAVE_EXTCONF_H" if $defs.empty? # needed so create_header works