Fix looking at CFLAGS

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "gcc"
-version = "0.3.0"
+version = "0.3.1"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/alexcrichton/gcc-rs"

src/lib.rs

@@ -270,7 +270,8 @@ fn ar(target: &str) -> String {
 
 fn cflags() -> Vec<String> {
     get_var("CFLAGS").unwrap_or(String::new())
-       .split(|c: char| c.is_whitespace()).map(|s| s.to_string())
+       .split(|c: char| c.is_whitespace()).filter(|s| !s.is_empty())
+       .map(|s| s.to_string())
        .collect()
 }