[RFC] Enable dev flags when --enable-debug is supplied

I was looking at the CMake config and noticed the `USE_DEV_FLAGS` option which i'd never seen before.

It seems a useful thing for devs (enables `-Wall -Wextra -Werror`), so enabling it with `--enable-debug` seems a solid move.

Tested by adding `int x;` in a random method and observing following build failure:

```
Bens-MBP:build ben$ ninja
[2/4] Building CXX object ycm/CMakeFiles/ycm_core.dir/Candidate.cpp.o
FAILED: ycm/CMakeFiles/ycm_core.dir/Candidate.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++   -DBOOST_THREAD_DONT_USE_CHRONO -DUSE_CLANG_COMPLETER -DYCMD_CORE_VERSION=25 -Dycm_core_EXPORTS -I/Users/ben/.vim/bundle/YouCompleteMe-Clean/third_party/ycmd/cpp/ycm -I/Users/ben/.vim/bundle/YouCompleteMe-Clean/third_party/ycmd/cpp/ycm/ClangCompleter -isystem /Users/ben/.vim/bundle/YouCompleteMe-Clean/third_party/ycmd/cpp/BoostParts -isystem /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/include/python2.7 -isystem ycm/../clang+llvm-3.9.0-x86_64-apple-darwin/include -stdlib=libc++ -fcolor-diagnostics -Wall -Wextra -Werror -Wc++98-compat -g -fPIC   -std=c++0x -MD -MT ycm/CMakeFiles/ycm_core.dir/Candidate.cpp.o -MF ycm/CMakeFiles/ycm_core.dir/Candidate.cpp.o.d -o ycm/CMakeFiles/ycm_core.dir/Candidate.cpp.o -c /Users/ben/.vim/bundle/YouCompleteMe-Clean/third_party/ycmd/cpp/ycm/Candidate.cpp
/Users/ben/.vim/bundle/YouCompleteMe-Clean/third_party/ycmd/cpp/ycm/Candidate.cpp:56:7: error: unused variable 'x' [-Werror,-Wunused-variable]
  int x;
      ^
1 error generated.
ninja: build stopped: subcommand failed.
```

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[READY] Update ycmd

# PR Prelude

Thank you for working on YCM! :)

**Please complete these steps and check these boxes (by putting an `x` inside
the brackets) _before_ filing your PR:**

- [X] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [X] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [X] I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.

> ycmd tests itself

- [X] **I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.**

# Why this change is necessary and useful

A number of enhancements have been made in the server. They seem stable in my testing. Let's push them live now.

Includes all of the following PRs:

* Auto merge of ycm-core/ycmd#631 - vheon:refactor-request-wrap, r=micbou: [READY] Add 'first_filetype' to RequestWrap
* Auto merge of ycm-core/ycmd#634 - vheon:fix-previous-identifier, r=micbou: [READY] Avoid to wrap around when searching for previous identifier
* Auto merge of ycm-core/ycmd#635 - micbou:resource-warning, r=Valloric: [READY] Fix warnings when resetting process handle
* Auto merge of ycm-core/ycmd#637 - vheon:refactor-tags-file, r=micbou: [READY] Extract filtering of tags as a generator
* Auto merge of ycm-core/ycmd#639 - mixedCase:tern_0.20, r=micbou: Bump Tern to version 0.20
* Auto merge of ycm-core/ycmd#640 - vheon:test-identifier-completer, r=micbou: [READY] Additional test for the identifier completer
* Auto merge of ycm-core/ycmd#636 - puremourning:coverage-c++, r=micbou: [READY] Coverage for c++ code
* Auto merge of ycm-core/ycmd#641 - micbou:remove-tests-folder, r=vheon: [READY] Remove unused tests folderS
* Auto merge of ycm-core/ycmd#643 - micbou:javascript-identifiers, r=Valloric: [READY] Add regex for JavaScript and TypeScript identifiers
* Auto merge of ycm-core/ycmd#644 - micbou:flake8, r=Valloric: [READY] Fix flake8 errors
* Auto merge of ycm-core/ycmd#645 - micbou:global-config-exception, r=Valloric: [READY] Catch exceptions from global extra conf
* Auto merge of ycm-core/ycmd#649 - micbou:codecov, r=puremourning: [READY] Use codecov bash uploader on Travis
* Auto merge of ycm-core/ycmd#652 - micbou:remove-executable-mode, r=Valloric: [READY] Remove executable mode
* Auto merge of ycm-core/ycmd#650 - vheon:remove-unused-assert, r=micbou: Remove unused CustomAssert
* Auto merge of ycm-core/ycmd#651 - micbou:global-extra-conf-reuse, r=Valloric: [READY] Do not reload extra conf file
* Auto merge of ycm-core/ycmd#657 - micbou:gocode-submodule, r=Valloric: [READY] Update Gocode submodule (Fixes #2449)
* Auto merge of ycm-core/ycmd#665 - micbou:flake8, r=micbou: [READY] Do not disable Flake8 on Python 2.6 Travis but restrict its version
* Auto merge of ycm-core/ycmd#667 - haifengkao:SupportObjcImportModule, r=Valloric: Support objc import module
* Auto merge of ycm-core/ycmd#677 - puremourning:dev-flags, r=micbou: [READY] Enable dev flags when --enable-debug is supplied

[cont]: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md
[code]: https://github.com/Valloric/YouCompleteMe/blob/master/CODE_OF_CONDUCT.md

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