Bug 1161339 - Check for the rust compiler. r=ted

Add a configure switch --enable-rust to control compilation
of rust source.

This serves as a basic conditional for experimentation with
gecko code written in rust. It presumes we can use a rustc
installed on the system, e.g. by the developer or by tooltool.

Requesting rust support without making rustc available will fail
since we don't want the build configuration to change based on
the available environment.

--HG--
extra : rebase_source : b3cf9fd07cf81df45513121a26bd5cc316b852d3

configure.in

@@ -429,6 +429,18 @@ MOZ_TOOL_VARIABLES
 
 MOZ_CHECK_COMPILER_WRAPPER
 
+MOZ_PATH_PROG(RUSTC, rustc)
+MOZ_ARG_ENABLE_BOOL([rust],
+                    [  --enable-rust  Include rust language sources],
+                    [MOZ_RUST=1],
+                    [MOZ_RUST= ])
+if test -z "$RUSTC" -a -n "$MOZ_RUST"; then
+  AC_MSG_ERROR([Rust compiler not found.
+    To compile rust language sources, you must have 'rustc' in your path.
+    See http://www.rust-lang.org/ for more information.])
+fi
+AC_SUBST(MOZ_RUST)
+
 dnl ========================================================
 dnl Check for MacOS deployment target version
 dnl ========================================================