Fix linker problems when compiling as C++.

When compiling compat-5.3.c as C++, the Lua symbols may still have
C-linkage. Reference those symbols as `extern "C"` by default and add an
option for normal (C++) linkage.

Author: siffiejoe

c-api/compat-5.3.c

@@ -3,8 +3,6 @@
 #include <string.h>
 #include <ctype.h>
 #include <errno.h>
-#include <lua.h>
-#include <lauxlib.h>
 #include "compat-5.3.h"
 
 /* don't compile it again if it already is included via compat53.h */

c-api/compat-5.3.h

@@ -4,8 +4,14 @@
 #include <stddef.h>
 #include <limits.h>
 #include <string.h>
+#if defined( __cplusplus ) && !defined( COMPAT53_LUA_CPP )
+extern "C" {
+#endif
 #include <lua.h>
 #include <lauxlib.h>
+#if defined( __cplusplus ) && !defined( COMPAT53_LUA_CPP )
+}
+#endif
 
 
 #if defined(COMPAT53_PREFIX)