travis: enable typecheck-gcc warnings

[MarcelRaad]

• switch debug and release configurations so that we get an optimized
  build with GCC 4.3+ as required by typecheck-gcc
• enable compiler warnings also for release builds

Testing if this works before merging #1592.

[mention-bot]

@MarcelRaad, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bagder, @sieben and @jactry to be potential reviewers.

[coveralls]

Coverage increased (+0.01%) to 73.801% when pulling 64083bc34b8ee0307db27832110c23ec76bfb690 on MarcelRaad:travis_typecheck into 651e04c on curl:master.

[MarcelRaad]

When switching debug and release and enabling warnings for release, the warnings showed up in the build log, but the build was green anyway. Testing --enable-werror for release now (without --enable-warnings).

[MarcelRaad]

Ah, as the warnings happen to be in a test, for which --enable-werror has no effect, the build still succeeds.

[coveralls]

Coverage decreased (-0.01%) to 73.777% when pulling 423d0e4 on MarcelRaad:travis_typecheck into 651e04c on curl:master.

[MarcelRaad]

So this is the version I'd like to merge. It would at least error out on typecheck-gcc warnings (and other warnings enabled by default) in libcurl and the curl tool.