Fix build on Linux with Ruby 2.1
There appears to be a conflict with a definition of `UChar` conflicting
with Ruby's. We can avoid this by defining
`ONIG_ESCAPE_UCHAR_COLLISION`.
@@ -33,6 +33,7 @@ static ID parent;
/* Backwards compatibility to Ruby 2.1.0 */
#if RUBY_API_VERSION_CODE < 20200
+#define ONIG_ESCAPE_UCHAR_COLLISION 1
#include <ruby/encoding.h>
static VALUE rb_utf8_str_new(const char *str, long length) {
