Fix flycheck clang args loading

[zhengyangfeng00]

Trying to resolve #1258

[StreakyCobra]

FTR this PR has been tried by 2 persons on #1258 and reported it to be successfully solving it.

@zhengyangfeng00 Can you rebase it on develop to have a chance of being merged.

[pszynk]

this solved the problem with loading .clang_complete file.

When merging note that function add-to-hooks was renamed to spacemacs/add-to-hooks

Also i think that the regexp for replacing relative path with absolute ones is not exactly right, two points.

1. .clang_complete file is said to contain one clang flag per line, maybe one option per line would be more precise. That means that -isystem <some dir> should probably be one line in the file, but the regexp assumes it's split in two lines. Maybe it's minor thing, but it can be misleading. As a reference here's doc for a vim plugin that uses .clang_complete file link.

2. this is actually an error. Replacing paths doesn't take into account that the line can look like this:

-I../<some_file>

it takes the first . , applies function expand-file-name with path/to/.clang_complete and replaces first . with the result. Only we are left with the second . and in the end it looks like this:

-I</absolute/path/to/.clang_complete>./<some_file> (note left .)

I'd be happy to propose better solution, but I'm still a bit green in elisp.

[TheBB]

Hi guys.

Was about to merge this but the repository has been deleted. Therefore I've opened #3819 with essentially the same changes. I hope that is okay with you @zhengyangfeng00 (you will get credit in the changelog). Since I don't use clang that much I would appreciate some code review over there. In particular your comments @pszynk... I'm not sure how they would apply, exactly.