If the CC environment variable is set, use its value instead of hardcoding "cc".
This fixes crbug.com/1090829.

diff --git a/src/librustc_codegen_ssa/back/link.rs b/src/librustc_codegen_ssa/back/link.rs
index 8725bfa..5984569 100644
--- a/src/librustc_codegen_ssa/back/link.rs
+++ b/src/librustc_codegen_ssa/back/link.rs
@@ -752,12 +752,12 @@ fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
             (Some(linker), Some(flavor)) => Some((linker, flavor)),
             // only the linker flavor is known; use the default linker for the selected flavor
             (None, Some(flavor)) => Some((
-                PathBuf::from(match flavor {
+                match flavor {
                     LinkerFlavor::Em => {
                         if cfg!(windows) {
-                            "emcc.bat"
+                            "emcc.bat".into()
                         } else {
-                            "emcc"
+                            "emcc".into()
                         }
                     }
                     LinkerFlavor::Gcc => {
@@ -768,16 +768,19 @@ fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
                             // and many modern illumos distributions today
                             // ship GCC as "gcc" without also making it
                             // available as "cc".
-                            "gcc"
+                            "gcc".into()
                         } else {
-                            "cc"
+                            match env::var_os("CC") {
+                                Some(path) => path.into(),
+                                None => "cc".into()
+                            }
                         }
                     }
-                    LinkerFlavor::Ld => "ld",
-                    LinkerFlavor::Msvc => "link.exe",
-                    LinkerFlavor::Lld(_) => "lld",
-                    LinkerFlavor::PtxLinker => "rust-ptx-linker",
-                }),
+                    LinkerFlavor::Ld => "ld".into(),
+                    LinkerFlavor::Msvc => "link.exe".into(),
+                    LinkerFlavor::Lld(_) => "lld".into(),
+                    LinkerFlavor::PtxLinker => "rust-ptx-linker".into(),
+                },
                 flavor,
             )),
             (Some(linker), None) => {