Allow users to opt out of the default kernel. (#958)

* Allow users to opt out of the default kernel.

* document IJULIA_NODEFAULTKERNEL

Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>

Author: vchuravy

deps/build.jl

@@ -1,8 +1,10 @@
 using Conda
 
-# Install Jupyter kernel-spec file.
-include("kspec.jl")
-kernelpath = installkernel("Julia", "--project=@.")
+if !haskey(ENV, "IJULIA_NODEFAULTKERNEL")
+    # Install Jupyter kernel-spec file.
+    include("kspec.jl")
+    kernelpath = installkernel("Julia", "--project=@.")
+end
 
 # make it easier to get more debugging output by setting JULIA_DEBUG=1
 # when building.

docs/src/manual/installation.md

@@ -73,6 +73,7 @@ installkernel("Julia (4 threads)", env=Dict("JULIA_NUM_THREADS"=>"4"))
 ```
 The `env` keyword should be a `Dict` mapping environment variables to values.
 
+To *prevent* IJulia from installing a default kernel when the package is built, define the `IJULIA_NODEFAULTKERNEL` environment variable before adding/building IJulia.
 
 ## Low-level Information