travis: attempt to fix unlikely build issue

This is a long chain of errors, and there are likely other changes that could
be made to cope in other places along that chain.

If depends don't build successfully, don't bother trying again for the sake of
better logging. That's likely to hurt more than help. In this case, qt build
failed, and on the second attempt, it appeared to be successful. However, due
to a bad object from an internal gcc error on the first build, the resulting
lib was unusable. This caused bitcoin-qt to not be built, and tests and
packaging which expected bitcoin-qt to be there failed.

The root cause:
Mingw is especially crashy when using -jX, likely compounded by low-memory
environments. I've seen multiple problems with this combo in Gitian as well.

In this case:
i686-w64-mingw32-g++: internal compiler error: Killed (program cc1plus)
...
make[3]: *** [.obj/release/qdrawhelper.o] Error 4

The workaround:
Bump Travis down to using -j2 by default. Additionaly, enable --with-gui for
the windows builds. This will cause configure to fail if qt is not working
while also testing the config flag.

Other failures which may be worth revisiting separately:
- If a depends package fails, maybe remove the workdir so that it doesn't taint
  subsequent runs
- See if there's anything repeatable about the ICE when building qt

.travis.yml

@@ -35,9 +35,9 @@ matrix:
     - compiler: ": Cross-Mac"
       env: HOST=x86_64-apple-darwin11 PACKAGES="gcc-multilib g++-multilib cmake libcap-dev libz-dev libbz2-dev" OSX_SDK=10.7 GOAL="deploy"
     - compiler: ": Win64"
-      env: HOST=x86_64-w64-mingw32 PACKAGES="nsis gcc-mingw-w64-x86-64 g++-mingw-w64-x86-64 binutils-mingw-w64-x86-64 mingw-w64-dev wine bc" RUN_TESTS=true GOAL="deploy"
+      env: HOST=x86_64-w64-mingw32 PACKAGES="nsis gcc-mingw-w64-x86-64 g++-mingw-w64-x86-64 binutils-mingw-w64-x86-64 mingw-w64-dev wine bc" RUN_TESTS=true GOAL="deploy" BITCOIN_CONFIG="--enable-gui" MAKEJOBS="-j2"
     - compiler: ": Win32"
-      env: HOST=i686-w64-mingw32 PACKAGES="nsis gcc-mingw-w64-i686 g++-mingw-w64-i686 binutils-mingw-w64-i686 mingw-w64-dev wine bc" RUN_TESTS=true GOAL="deploy"
+      env: HOST=i686-w64-mingw32 PACKAGES="nsis gcc-mingw-w64-i686 g++-mingw-w64-i686 binutils-mingw-w64-i686 mingw-w64-dev wine bc" RUN_TESTS=true GOAL="deploy" BITCOIN_CONFIG="--enable-gui" MAKEJOBS="-j2"
 install:
     - if [ -n "$PACKAGES" ]; then travis_retry sudo apt-get update; fi
     - if [ -n "$PACKAGES" ]; then travis_retry sudo apt-get install --no-upgrade -qq $PACKAGES; fi
@@ -46,7 +46,7 @@ before_script:
     - mkdir -p depends/SDKs depends/sdk-sources
     - if [ -n "$OSX_SDK" -a ! -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then wget $SDK_URL/MacOSX${OSX_SDK}.sdk.tar.gz -O depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi
     - if [ -n "$OSX_SDK" -a -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then tar -C depends/SDKs -xf depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi
-    - make $MAKEJOBS -C depends HOST=$HOST $DEP_OPTS || (echo "Build failure. Verbose build follows." && make -C depends V=1 HOST=$HOST $DEP_OPTS)
+    - make $MAKEJOBS -C depends HOST=$HOST $DEP_OPTS
 script:
     - if [ -n "$USE_SHELL" ]; then export CONFIG_SHELL="$USE_SHELL"; fi
     - OUTDIR=$BASE_OUTDIR/$TRAVIS_PULL_REQUEST/$TRAVIS_JOB_NUMBER-$HOST