Fixed case when compile db is not at project root.

Added -isystem flag awareness.

plugin/clang.vim

@@ -901,6 +901,7 @@ python << endpython
 import vim
 import re
 import json
+from os import path
 
 current = vim.eval("expand('%:p')")
 ccd = vim.eval("l:ccd")
@@ -915,8 +916,12 @@ with open(ccd) as database:
     dmatch = re.search(r'(.*)\.(\w+)$', d['file'])
 
     if fmatch.group(1) == dmatch.group(1):
-      for result in re.finditer(r'-[ID]\s*[^\s]+', d['command']):
+      for result in re.finditer(r'-D\s*[^\s]+', d['command']):
         opts.append(result.group(0))
+      for result in re.finditer(r'-isystem\s*[^\s]+', d['command']):
+        opts.append(result.group(0))
+      for result in re.finditer(r'-I\s*([^\s]+)', d['command']):
+        opts.append('-I' + path.join(d['directory'], result.group(1)))
       break
 
 vim.command("let l:clang_options = '" + ' '.join(opts) + "'")