Define rustc's host triple at compile time

This way a cross-compiled rustc's answer to host_triple() is correct. The return
value of host_triple() reflects the actual host triple that the compiler was
build for, not the triple the compiler is being built on

Author: alexcrichton

mk/target.mk

@@ -62,6 +62,10 @@ $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_RUSTLLVM_$(3)): \
 	@$$(call E, cp: $$@)
 	$$(Q)cp $$< $$@
 
+# This is the compile-time target-triple for the compiler. For the compiler at
+# runtime, this should be considered the host-triple. More explanation for why
+# this exists can be found on issue #2400
+$$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_LIBRUSTC_$(3)): export CFG_COMPILER_TRIPLE := $(2)
 $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_LIBRUSTC_$(3)):		\
 		$$(COMPILER_CRATE) $$(COMPILER_INPUTS)		\
                 $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_LIBSYNTAX_$(3)) \

src/librustc/driver/driver.rs

@@ -516,15 +516,15 @@ pub fn host_triple() -> ~str {
     // idea of the host triple is the same as for the set of libraries we've
     // actually built.  We can't just take LLVM's host triple because they
     // normalize all ix86 architectures to i386.
-
-    // FIXME (#2400): Instead of grabbing the host triple we really should
-    // be grabbing (at compile time) the target triple that this rustc is
-    // built with and calling that (at runtime) the host triple.
-    let ht = env!("CFG_BUILD_TRIPLE");
+    //
+    // Instead of grabbing the host triple (for the current host), we grab (at
+    // compile time) the target triple that this rustc is built with and
+    // calling that (at runtime) the host triple.
+    let ht = env!("CFG_COMPILER_TRIPLE");
     return if ht != ~"" {
             ht
         } else {
-            fail!("rustc built without CFG_BUILD_TRIPLE")
+            fail!("rustc built without CFG_COMPILER_TRIPLE")
         };
 }
 
@@ -534,15 +534,15 @@ pub fn host_triple() -> ~str {
     // idea of the host triple is the same as for the set of libraries we've
     // actually built.  We can't just take LLVM's host triple because they
     // normalize all ix86 architectures to i386.
-
-    // FIXME (#2400): Instead of grabbing the host triple we really should
-    // be grabbing (at compile time) the target triple that this rustc is
-    // built with and calling that (at runtime) the host triple.
-    let ht = env!("CFG_BUILD_TRIPLE");
+    //
+    // Instead of grabbing the host triple (for the current host), we grab (at
+    // compile time) the target triple that this rustc is built with and
+    // calling that (at runtime) the host triple.
+    let ht = env!("CFG_COMPILER_TRIPLE");
     return if ht != "" {
             ht.to_owned()
         } else {
-            fail!("rustc built without CFG_BUILD_TRIPLE")
+            fail!("rustc built without CFG_COMPILER_TRIPLE")
         };
 }