fix: use pkg-config to find Lua library (#42)

Use pkg-config with fallbacks to resolve the correct library name
across platforms (lua on macOS, lua5.4 on Ubuntu/Debian).
Add installation instructions for common platforms.

Author: veelenga

README.md

@@ -8,7 +8,23 @@ Crystal bindings to Lua. Supports [Lua 5.4](http://www.lua.org/ftp/) and higher
 
 ## Installation
 
-Add this to your application's `shard.yml`:
+Lua development libraries must be installed on your system:
+
+```bash
+# macOS
+brew install lua
+
+# Ubuntu/Debian
+sudo apt install liblua5.4-dev
+
+# Arch Linux
+sudo pacman -S lua
+
+# Fedora
+sudo dnf install lua-devel
+```
+
+Then add this to your application's `shard.yml`:
 
 ```yaml
 dependencies:

src/lua/liblua.cr

@@ -1,4 +1,4 @@
-@[Link("lua")]
+@[Link(ldflags: "`pkg-config --libs lua 2>/dev/null || pkg-config --libs lua5.4 2>/dev/null || echo -llua`")]
 lib LibLua
   type State = Void*