Docs: Add OSX instructions for building You-Complete-Me.

Without the --system-clang flag, ycmd fails to find the system headers,
which severely limits its usefulness. The flag is documented as
unsupported, but it's recommended by the maintainers on OSX:
https://github.com/Valloric/YouCompleteMe/issues/2536#issuecomment-333909807

Change-Id: I93aeb12656d32e0fa2099aa0c703fcb6f7154bfb
Reviewed-on: https://chromium-review.googlesource.com/925102Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Jianpeng Chao <chaopeng@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537864}

docs/atom.md

@@ -46,6 +46,12 @@ $ cd ~/.ycmd
 $ ./build.py --clang-completer
 ```
 
+On Mac, replace the last command above with the following.
+
+```
+$ ./build.py --clang-completer --system-libclang
+```
+
 ## JavaScript lint
 
 Install JavaScript linter for Blink layout tests.

docs/vscode.md

@@ -88,6 +88,12 @@ $ ./build.py --clang-completer
 If it fails with "Your C++ compiler does NOT fully support C++11." but you know
 you have a good compiler, hack cpp/CMakeLists.txt to set CPP11_AVAILABLE true.
 
+On Mac, replace the last command above with the following.
+
+```
+$ ./build.py --clang-completer --system-libclang
+```
+
 The following extensions might be useful for you as well:
 
 *   ***Annotator*** -