Build configuration via defines

[Daurnimator <quae@...>]

Hi Roberto,

Have you considered allowing (some) things in luaconf.h to be easily
configurable from compiler command line options?
This usually means allowing configuration via `-D` switches.
To make this possible, luaconf.h should only define things if they are
not already defined.

e.g,
Currently there is:

#define LUA_ROOT "/usr/local/"


It would be easier for packaging if this was instead:

#ifndef LUA_ROOT
#define LUA_ROOT "/usr/local/"
#endif


I assume this doesn't occur today as luaconf.h is installed as a
header onto end user systems.
There are some ways around this:
  - some of the defines (such as LUA_ROOT) probably don't need to be
available for compiling modules
  - for values that do need to exist at runtime, they could be made
available through externs, or some new function
lua_compilation_flags();