Add the build date to the reported version. #21957

rustc --version says

```
rustc 1.0.0-dev (d0e82a6 2015-02-05) (built 2015-02-11)
```

mk/main.mk

@@ -61,13 +61,16 @@ SPACE :=
 SPACE +=
 ifneq ($(CFG_GIT),)
 ifneq ($(wildcard $(subst $(SPACE),\$(SPACE),$(CFG_GIT_DIR))),)
-    CFG_VER_DATE = $(shell git --git-dir='$(CFG_GIT_DIR)' log -1 --pretty=format:'%ci')
+    CFG_VER_DATE = $(shell git --git-dir='$(CFG_GIT_DIR)' log -1 --date=short --pretty=format:'%cd')
     CFG_VER_HASH = $(shell git --git-dir='$(CFG_GIT_DIR)' rev-parse HEAD)
     CFG_SHORT_VER_HASH = $(shell git --git-dir='$(CFG_GIT_DIR)' rev-parse --short=9 HEAD)
     CFG_VERSION += ($(CFG_SHORT_VER_HASH) $(CFG_VER_DATE))
 endif
 endif
 
+CFG_BUILD_DATE = $(shell date +%F)
+CFG_VERSION += (built $(CFG_BUILD_DATE))
+
 # Windows exe's need numeric versions - don't use anything but
 # numbers and dots here
 CFG_VERSION_WIN = $(CFG_RELEASE_NUM)
@@ -317,6 +320,7 @@ endif
 ifdef CFG_VER_HASH
 export CFG_VER_HASH
 endif
+export CFG_BUILD_DATE
 export CFG_VERSION
 export CFG_VERSION_WIN
 export CFG_RELEASE

src/librustc_driver/lib.rs

@@ -276,6 +276,10 @@ pub fn commit_date_str() -> Option<&'static str> {
     option_env!("CFG_VER_DATE")
 }
 
+pub fn build_date_str() -> Option<&'static str> {
+    option_env!("CFG_BUILD_DATE")
+}
+
 /// Prints version information and returns None on success or an error
 /// message on panic.
 pub fn version(binary: &str, matches: &getopts::Matches) {
@@ -287,6 +291,7 @@ pub fn version(binary: &str, matches: &getopts::Matches) {
         println!("binary: {}", binary);
         println!("commit-hash: {}", unw(commit_hash_str()));
         println!("commit-date: {}", unw(commit_date_str()));
+        println!("build-date: {}", unw(build_date_str()));
         println!("host: {}", config::host_triple());
         println!("release: {}", unw(release_str()));
     }