fix(ruby): Fixed include paths for Ruby Windows builds

ruby/ext/google/protobuf_c/extconf.rb

@@ -7,19 +7,18 @@ ext_name = "google/protobuf_c"
 dir_config(ext_name)
 
 if RUBY_PLATFORM =~ /darwin/ || RUBY_PLATFORM =~ /linux/ || RUBY_PLATFORM =~ /freebsd/
-  $CFLAGS += " -std=gnu99 -O3 -DNDEBUG -fvisibility=hidden -Wall -Wsign-compare -Wno-declaration-after-statement -I$(srcdir)/third_party/utf8_range"
+  $CFLAGS += " -std=gnu99 -O3 -DNDEBUG -fvisibility=hidden -Wall -Wsign-compare -Wno-declaration-after-statement"
 else
   $CFLAGS += " -std=gnu99 -O3 -DNDEBUG"
 end
 
-
 if RUBY_PLATFORM =~ /linux/
   # Instruct the linker to point memcpy calls at our __wrap_memcpy wrapper.
   $LDFLAGS += " -Wl,-wrap,memcpy"
 end
 
 $VPATH << "$(srcdir)/third_party/utf8_range"
-$INCFLAGS << "$(srcdir)/third_party/utf8_range"
+$INCFLAGS += " -I$(srcdir)/third_party/utf8_range"
 
 $srcs = ["protobuf.c", "convert.c", "defs.c", "message.c",
          "repeated_field.c", "map.c", "ruby-upb.c", "wrap_memcpy.c",