Bug 1208566 - require Rust 1.5 for --enable-rust builds; r=mshal

Rust 1.5 features fine-grained dependency tracking that we need for
proper incremental builds.  As a side effect, it also features choosing
between the builtin-to-Rust jemalloc and the system allocator, which
Gecko needs for interoperation.  (Gecko needs to make Rust use the
system allocator, which then gets redirected into Gecko's copy of
jemalloc if necessary.)  It's not great to require an unstable Rust
target here, but we need to upgrade at some point anyway, so we might as
well make people aware of the upgrade requirement sooner rather than
later.

build/autoconf/rust.m4

@@ -25,10 +25,10 @@ AC_DEFUN([MOZ_RUST_SUPPORT], [
   fi
   if test -n "$MOZ_RUST" && test -z "$_RUSTC_MAJOR_VERSION" -o \
     "$_RUSTC_MAJOR_VERSION" -lt 1 -o \
-    \( "$_RUSTC_MAJOR_VERSION" -eq 1 -a "$_RUSTC_MINOR_VERSION" -lt 4 \); then
+    \( "$_RUSTC_MAJOR_VERSION" -eq 1 -a "$_RUSTC_MINOR_VERSION" -lt 5 \); then
     AC_MSG_ERROR([Rust compiler ${RUSTC_VERSION} is too old.
       To compile Rust language sources please install at least
-      version 1.4 of the 'rustc' toolchain and make sure it is
+      version 1.5 of the 'rustc' toolchain and make sure it is
       first in your path.
       You can verify this by typing 'rustc --version'.])
   fi