Fix #356 Don't complain about #pragma once in C++ headers

autoload/ale/handlers/gcc.vim

@@ -3,6 +3,8 @@ scriptencoding utf-8
 " Description: This file defines a handler function which ought to work for
 " any program which outputs errors in the format that GCC uses.
 
+let s:pragma_error = '#pragma once in main file'
+
 function! s:AddIncludedErrors(output, include_lnum, include_lines) abort
     if a:include_lnum > 0
         call add(a:output, {
@@ -92,6 +94,11 @@ function! ale#handlers#gcc#HandleGCCFormat(buffer, lines) abort
             let l:include_lines = []
             let l:included_filename = ''
 
+            if s:IsHeaderFile(bufname(bufnr('')))
+            \&& l:match[5][:len(s:pragma_error) - 1] ==# s:pragma_error
+                continue
+            endif
+
             call add(l:output, {
             \   'lnum': l:match[2] + 0,
             \   'col': l:match[3] + 0,

test/handler/test_gcc_handler.vader

@@ -77,3 +77,20 @@ Execute(GCC versions should be parsed correctly):
   \ 'This is free software; see the source for copying conditions.  There is NO',
   \ 'warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.',
   \])
+
+Execute(The GCC handler shouldn't complain about #pragma once for headers):
+  silent file! test.h
+
+  AssertEqual
+  \ [],
+  \ ale#handlers#gcc#HandleGCCFormat(347, [
+  \   '<stdin>:1:1: warning: #pragma once in main file [enabled by default]',
+  \ ])
+
+  silent file! test.hpp
+
+  AssertEqual
+  \ [],
+  \ ale#handlers#gcc#HandleGCCFormat(347, [
+  \   '<stdin>:1:1: warning: #pragma once in main file [enabled by default]',
+  \ ])