Add -fPIC and -lpthread (see http://lua-users.org/lists/lua-l/2015-04/msg00010.html)

diff -Nur lua-5.4.2.old/src/Makefile lua-5.4.2/src/Makefile
--- lua-5.4.2.old/src/Makefile	2015-01-05 14:04:52.000000000 -0200
+++ lua-5.4.2/src/Makefile	2015-04-04 15:21:07.841376633 -0300
@@ -121,7 +121,7 @@
 	$(MAKE) $(ALL) SYSCFLAGS="-DLUA_USE_LINUX" SYSLIBS="-Wl,-E -ldl"
 
 linux-readline:
-	$(MAKE) $(ALL) SYSCFLAGS="-DLUA_USE_LINUX -DLUA_USE_READLINE" SYSLIBS="-Wl,-E -ldl -lreadline"
+	$(MAKE) $(ALL) SYSCFLAGS="-DLUA_USE_LINUX -DLUA_USE_READLINE -fPIC" SYSLIBS="-Wl,-E -ldl -lreadline -lpthread"
 
 Darwin macos macosx:
 	$(MAKE) $(ALL) SYSCFLAGS="-DLUA_USE_MACOSX -DLUA_USE_READLINE" SYSLIBS="-lreadline"