lua.c loads 'readline' dynamically

(See comments in luaconf.h.) This change allows easier compilation,
as Lua compiles and works even if the package 'readline' is absent
from the system. Moreover, non-interactive uses don't load the library,
making the stand-alone slightly faster for small loads.

makefile

@@ -70,9 +70,9 @@ LOCAL = $(TESTS) $(CWARNS)
 
 
 # enable Linux goodies
-MYCFLAGS= $(LOCAL) -std=c99 -DLUA_USE_LINUX -DLUA_USE_READLINE
+MYCFLAGS= $(LOCAL) -std=c99 -DLUA_USE_LINUX
 MYLDFLAGS= $(LOCAL) -Wl,-E
-MYLIBS= -ldl -lreadline
+MYLIBS= -ldl
 
 
 CC= gcc