Allow installing package without R installation

This is in case the user wants to specify and installation via
Preferences.jl after installation. An error message is printed at import
time if no installation is available.

deps/build.jl

@@ -25,6 +25,7 @@ try
         @info "Using previously configured R at $Rhome with libR in $libR."
     else
         Rhome = get(ENV, "R_HOME", "")
+        libR = nothing
         if Rhome == "*"
             # install with Conda
             @info "Installing R via Conda.  To use a different R installation,"*
@@ -54,16 +55,30 @@ try
                 end
             end
 
-            isempty(Rhome) && error("R cannot be found. Set the \"R_HOME\" environment variable to re-run Pkg.build(\"RCall\").")
-            libR = locate_libR(Rhome)
+            if !isempty(Rhome)
+                libR = locate_libR(Rhome)
+            end
         end
 
-        @info "Using R at $Rhome and libR at $libR."
-        if DepFile.Rhome != Rhome || DepFile.libR != libR
+        if isempty(Rhome)
+            @info (
+                "No R installation found. " *
+                "You will not be able to import RCall without " *
+                "providing values for its preferences Rhome and libR."
+            )
             open(depfile, "w") do f
-                println(f, "const Rhome = \"", escape_string(Rhome), '"')
-                println(f, "const libR = \"", escape_string(libR), '"')
-                println(f, "const conda_provided_r = $(conda_provided_r)")
+                println(f, "const Rhome = nothing")
+                println(f, "const libR = nothing")
+                println(f, "const conda_provided_r = nothing")
+            end
+        else
+            @info "Using R at $Rhome and libR at $libR."
+            if DepFile.Rhome != Rhome || DepFile.libR != libR
+                open(depfile, "w") do f
+                    println(f, "const Rhome = \"", escape_string(Rhome), '"')
+                    println(f, "const libR = \"", escape_string(libR), '"')
+                    println(f, "const conda_provided_r = $(conda_provided_r)")
+                end
             end
         end
     end

src/RCall.jl

@@ -41,6 +41,13 @@ else
     const depfile = joinpath(dirname(@__FILE__),"..","deps","deps.jl")
     if isfile(depfile)
         include(depfile)
+        if Rhome === nothing
+            error(
+                "No R installation was detected as RCall installation time. " *
+                "Please provided the location of R by setting the Rhome and libR preferences or " *
+                "else set R_HOME='*' and rerun Pkg.build(\"RCall\") to use Conda.jl."
+            )
+        end
     else
         error("RCall not properly installed. Please run Pkg.build(\"RCall\")")
     end