Move file detection to ftdetect & fix global usage.

Use fallback to $GOROOT if g:golang_goroot variable is not set.

Author: rjohnsondev

Diff for: README.md

@@ -10,6 +10,12 @@ Just clone this repo into your bundles directory:
 
     git clone https://github.com/rjohnsondev/vim-compiler-go.git ~/.vim/bundle/vim-compiler-go
 
+Either ensure the $GOROOT environment variable is set, or otherwise set the
+g:golang_goroot variable in .vimrc to where your go installation can be found.
+This must be an absolute path:
+
+    let g:golang_goroot = "/home/richard/go"
+
 ### Manual installation
 
 Drop golang.vim in ~/.vim/compiler directory.

Diff for: ftdetect/go.vim

@@ -0,0 +1 @@
+autocmd FileType go compiler golang

Diff for: ftplugin/go.vim

@@ -1,2 +1,7 @@
-autocmd FileType go compiler golang
-let g:golang_goroot = $GOROOT
+if !empty($GOROOT) && !exists('g:golang_goroot')
+    let g:golang_goroot = $GOROOT
+endif
+if !exists('g:golang_goroot')
+    let g:golang_goroot = '/usr/bin'
+    echoerr 'Please set $GOROOT or g:golang_goroot'
+endif