Make the changes I proposed to PR #79

Allows people (maybe only I care about this?) to build rlua in weird
environments.

Cargo.toml

@@ -23,6 +23,13 @@ default = ["builtin-lua"]
 #   * LUA_EXTRASPACE is at least pointer sized and has at least pointer alignment.
 #   * LUAI_MAXSTACK is 1_000_000
 builtin-lua = ["cc"]
+# Uses pkg-config to find an appropriate lua 5.3 library to link with.  All of
+# the caveats about disabling the default builtin-lua feature apply here as
+# well.  If neither the builtin-lua nor the system-lua feature is enabled, then
+# no lua library will be linked at all and one must be linked with or built into
+# the final binary manually.  The builtin-lua and system-lua features are
+# mutually exclusive and enabling both will cause an error at build time.
+system-lua = ["pkg-config"]
 
 [dependencies]
 libc = { version = "0.2" }
@@ -31,7 +38,7 @@ compiletest_rs = { version = "0.3", optional = true }
 
 [build-dependencies]
 cc = { version = "1.0", optional = true }
-pkg-config = "0.3.11"
+pkg-config = { version = "0.3.11", optional = true }
 
 [dev-dependencies]
 rustyline = "1.0.0"

build.rs

@@ -1,10 +1,14 @@
 #[cfg(feature = "builtin-lua")]
 extern crate cc;
 
-#[cfg(not(feature = "builtin-lua"))]
+#[cfg(feature = "system-lua")]
 extern crate pkg_config;
 
 fn main() {
+    if cfg!(all(feature = "builtin-lua", feature = "system-lua")) {
+        panic!("cannot enable both builtin-lua and system-lua features when building rlua");
+    }
+
     #[cfg(feature = "builtin-lua")]
     {
         use std::env;
@@ -65,8 +69,12 @@ fn main() {
             .file("lua/lzio.c")
             .compile("liblua5.3.a");
     }
-    #[cfg(not(feature = "builtin-lua"))]
+
+    #[cfg(feature = "system-lua")]
     {
-        pkg_config::Config::new().atleast_version("5.3").probe("lua").unwrap();
+        pkg_config::Config::new()
+            .atleast_version("5.3")
+            .probe("lua")
+            .unwrap();
     }
 }