Merge bitcoin#10923: travis: Build with --enable-werror under OS X

a65e028 Build with --enable-werror under OS X (practicalswift)

Pull request description:

  Build with `--enable-werror` under OS X.

  As suggested by @TheBlueMatt in bitcoin#10866. This will allow catching violations using Travis CI which does a `clang` build for OS X.

Tree-SHA512: 326248897e0776106983e0824e7e80eee3c6e584a1d360f429c30f3375dad83ab4c360c86ab0729bd9ede747ea0caa13cd6a7c35072ed9b845362423c9c37a64

.travis.yml

@@ -34,7 +34,7 @@ env:
 # x86_64 Linux, No wallet
     - HOST=x86_64-unknown-linux-gnu PACKAGES="python3" DEP_OPTS="NO_WALLET=1" RUN_TESTS=true GOAL="install" BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports"
 # Cross-Mac
-    - HOST=x86_64-apple-darwin11 PACKAGES="cmake imagemagick libcap-dev librsvg2-bin libz-dev libbz2-dev libtiff-tools python-dev" BITCOIN_CONFIG="--enable-gui --enable-reduce-exports" OSX_SDK=10.11 GOAL="deploy"
+    - HOST=x86_64-apple-darwin11 PACKAGES="cmake imagemagick libcap-dev librsvg2-bin libz-dev libbz2-dev libtiff-tools python-dev" BITCOIN_CONFIG="--enable-gui --enable-reduce-exports --enable-werror" OSX_SDK=10.11 GOAL="deploy"
 
 before_install:
     - export PATH=$(echo $PATH | tr ':' "\n" | sed '/\/opt\/python/d' | tr "\n" ":" | sed "s|::|:|g")