Make building regex module optional

[bstaletic]

Two things:

1. Make regex optional with --no-regex/--regex
2. Move BuildRegexModule() to the end.

Both done as a way to allow ycmd to work even if regex does not compile on some system.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.88%. Comparing base (e6a0baa) to head (2344ca4).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1764   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          84       84           
  Lines        8465     8465           
  Branches      163      163           
=======================================
  Hits         8117     8117           
  Misses        298      298           
  Partials       50       50           

[puremourning]

nice, thanks!

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: 1 of 2 LGTMs obtained (waiting on @bstaletic)

[mergify]

Thanks for sending a PR!