Added MXNET_COMMIT env variable for upstream commit/tag (#260)

Setting the `MXNET_COMMIT` environment variable to a tag, branch or hash
will use that version of the upstream mxnet repo in building the libmxnet
shared object.

Additionally, the upstream submodules are cleared (and reinitialised)
every build to prevent some issues when submodules make changes
(squashes, etc).

deps/build.jl

@@ -5,7 +5,7 @@ import JSON
 # First try to detect and load existing libmxnet
 ################################################################################
 libmxnet_detected = false
-libmxnet_curr_ver = "master"
+libmxnet_curr_ver = get(ENV, "MXNET_COMMIT", "master")
 curr_win = "20170502"
 
 if haskey(ENV, "MXNET_HOME")
@@ -138,14 +138,15 @@ if !libmxnet_detected
       @build_steps begin
         BinDeps.DirectoryRule(_mxdir, @build_steps begin
           ChangeDirectory(_srcdir)
-          `git clone --recursive https://github.com/dmlc/mxnet`
+          `git clone https://github.com/dmlc/mxnet`
         end)
         @build_steps begin
           ChangeDirectory(_mxdir)
-          `git -C mshadow checkout -- make/mshadow.mk`
+          `git submodule deinit --force .`
           `git fetch`
           `git checkout $libmxnet_curr_ver`
-          `git submodule update --init`
+          `git submodule update --init --recursive`
+          `git -C mshadow checkout -- make/mshadow.mk`
           `make clean`
           `sed -i -s "s/MSHADOW_CFLAGS = \(.*\)/MSHADOW_CFLAGS = \1 $ilp64/" mshadow/make/mshadow.mk`
         end

docs/src/user-guide/install.md

@@ -21,10 +21,13 @@ MXNet.jl is built on top of [libmxnet](https://github.com/dmlc/mxnet).
 Upon installation, Julia will try to automatically download and build
 libmxnet.
 
-There are two environment variables that change this behaviour. If you
+There are three environment variables that change this behaviour. If you
 already have a pre-installed version of mxnet you can use `MXNET_HOME`
 to point the build-process in the right direction. If the automatic
 cuda detection fails you can also set `CUDA_HOME` to override the process.
+To control which version of libmxnet will be compiled, you can use the
+`MXNET_COMMIT` variable to point to either a version tag (e.g. `v0.10.0`), a
+branch name (e.g.  `master`) or a specific commit hash (e.g. `a0b1c2d3`).
 
 The libmxnet source is downloaded to `Pkg.dir("MXNet")/deps/src/mxnet`.
 The automatic build is using default configurations, with OpenCV disabled.