add ReadConfig() function for C/C++ checkers

autoload/syntastic/c.vim

@@ -65,6 +65,41 @@ function! syntastic#c#GetIncludeDirs(filetype)
     return join(map(s:Unique(include_dirs), '"-I" . v:val'), ' ')
 endfunction
 
+" read additional compiler flags from the given configuration file
+" the file format and its parsing mechanism is inspired by clang_complete
+function! syntastic#c#ReadConfig(file)
+    " search in the current file's directory upwards
+    let config = findfile(a:file, '.;')
+    if config == '' || !filereadable(config) | return '' | endif
+
+    " convert filename into absolute path
+    let filepath = substitute(fnamemodify(config, ':p:h'), '\', '/', 'g')
+
+    " try to read config file
+    try
+        let lines = map(readfile(config),
+                    \ 'substitute(v:val, ''\'', ''/'', ''g'')')
+    catch /E484/
+        return ''
+    endtry
+
+    let parameters = []
+    for line in lines
+        let matches = matchlist(line, '^\s*-I\s*\(\S\+\)')
+        if matches != [] && matches[1] != ''
+            " this one looks like an absolute path
+            if match(matches[1], '^\%(/\|\a:\)') != -1
+                call add(parameters, '-I' . matches[1])
+            else
+                call add(parameters, '-I' . filepath . '/' . matches[1])
+            endif
+        else
+            call add(parameters, line)
+        endif
+    endfor
+
+    return join(parameters, ' ')
+endfunction
 
 " search the first 100 lines for include statements that are
 " given in the handlers dictionary