HEADS UP: adding pthread flags to Lua 5.3.2

[Alexander Nasonov <alnsn%yandex.ru@localhost>]

Hi,

New version of Lua 5.3 is out and it's a good opportunity to build it
with pthread support.

I'm going to add the following fragment to lang/lua53/Makefile:

+PTHREAD_OPTS+=         native
+
+.include "../../mk/pthread.buildlink3.mk"
+
+.if defined(PTHREAD_TYPE) && ${PTHREAD_TYPE} == "native"
+CFLAGS+=       -pthread
+LDFLAGS+=      -pthread
+.endif

I likely need to modify bl3.mk file too but not completely sure
how exactly it should be changed. Lua itself doesn't use threading,
if some package links -llua5.3, it doesn't necessarily mean that
pthread support should be enforced.

Suggestions are welcome!

Alex