diff --git a/src/lib.rs b/src/lib.rs
index c27daed..819d4b9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+#![feature(if_let)]
+
 use std::os;
 use std::io::Command;
 use std::io::process::InheritFd;
@@ -7,12 +9,15 @@ use std::default::Default;
 pub struct Config {
     /// Directories where gcc will look for header files.
     pub include_directories: Vec<Path>,
+    /// Additional definitions (`-DKEY` or `-DKEY=VALUE`).
+    pub definitions: Vec<(String, Option<String>)>,
 }
 
 impl Default for Config {
     fn default() -> Config {
         Config {
             include_directories: Vec::new(),
+            definitions: Vec::new(),
         }
     }
 }
@@ -59,6 +64,14 @@ pub fn compile_library(output: &str, config: &Config, files: &[&str]) {
         cmd.arg("-I").arg(directory);
     }
 
+    for &(ref key, ref value) in config.definitions.iter() {
+        if let &Some(ref value) = value {
+            cmd.arg(format!("-D{}={}", key, value));
+        } else {
+            cmd.arg(format!("-D{}", key));
+        }
+    }
+
     let src = Path::new(os::getenv("CARGO_MANIFEST_DIR").unwrap());
     let dst = Path::new(os::getenv("OUT_DIR").unwrap());
     let mut objects = Vec::new();