Fix #269 Set $TMPDIR to a default value, if not set

plugin/ale.vim

@@ -25,6 +25,12 @@ if !s:has_features
     finish
 endif
 
+" Set the TMPDIR environment variable if it is not set automatically.
+" This can automatically fix some environments.
+if has('unix') && empty($TMPDIR)
+    let $TMPDIR = '/tmp'
+endif
+
 " This global variable is used internally by ALE for tracking information for
 " each buffer which linters are being run against.
 let g:ale_buffer_info = {}

test/test_tmpdir_init.vader

@@ -0,0 +1,2 @@
+Execute($TMPDIR should be set to a default value if unset):
+  AssertEqual '/tmp', $TMPDIR

test/vimrc

@@ -20,3 +20,7 @@ set foldnestmax=10
 set ttimeoutlen=0
 
 let g:mapleader=','
+
+" Clear the TMPDIR value for tests.
+" The plugin should set this to /tmp by default, which we will test.
+let $TMPDIR = ''