Switch Travis builds to Ubuntu Xenial (16.04) to pick up a newer MinGW. (#508)

- Newer MinGW fails with std::threads as it defaults to Windows threads -
  force POSIX threads via cmake/toolchain-cross-mingw.cmake.
- Android fails during CMake generation due to incompatible
  cmake/toolchain-cross-android.cmake.  Switch to maintained toolchain file
  inside Android NDK.
- Android toolchain file in 13b NDK is broken.  Upgrade to 19c NDK.
- Upgrade Android API level from 15 to 16 to match Gecko (and minimum
  supported by 19c NDK).

.travis.yml

@@ -1,10 +1,8 @@
 language: cpp
-dist: trusty
+dist: xenial
 sudo: required
 addons:
   apt:
-    sources:
-      - ubuntu-toolchain-r-test
     packages:
       - clang-3.6
       - g++-4.8
@@ -24,7 +22,7 @@ matrix:
     - env: C_COMPILER=gcc-4.8 CXX_COMPILER=g++-4.8
     - env: SCAN_BUILD=true
     - env: CROSS=mingw
-    - env: CROSS=android API_LEVEL=android-15
+    - env: CROSS=android API_LEVEL=android-16
     - os: osx
       osx_image: xcode6.4
     - os: osx
@@ -39,7 +37,7 @@ before_install:
       export SCAN_BUILD_PATH="$SCAN_BUILD_PATH -o scan_results";
     fi
   - if [[ $CROSS = "android" ]]; then
-      NDK=android-ndk-r13b;
+      NDK=android-ndk-r19c;
       curl -LO https://dl.google.com/android/repository/$NDK-linux-x86_64.zip;
       unzip -q $NDK-linux-x86_64.zip;
       rm $NDK-linux-x86_64.zip;
@@ -56,7 +54,7 @@ before_script:
       export TOOLCHAIN=-DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain-cross-$CROSS.cmake;
     fi
   - if [[ $CROSS = "android" ]]; then
-      export TOOLCHAIN="$TOOLCHAIN -DANDROID_NATIVE_API_LEVEL=$API_LEVEL";
+      export TOOLCHAIN="-DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake -DANDROID_NATIVE_API_LEVEL=$API_LEVEL";
     fi
   - mkdir build && cd build
   - $SCAN_BUILD_PATH cmake -DCMAKE_BUILD_TYPE=Debug -DSANITIZE_ADDRESS=$CMAKE_USE_ASAN $TOOLCHAIN ..;

cmake/toolchain-cross-mingw.cmake

@@ -3,8 +3,8 @@ SET(CMAKE_SYSTEM_NAME Windows)
 set(COMPILER_PREFIX "i686-w64-mingw32")
 
 find_program(CMAKE_RC_COMPILER NAMES ${COMPILER_PREFIX}-windres)
-find_program(CMAKE_C_COMPILER NAMES ${COMPILER_PREFIX}-gcc)
-find_program(CMAKE_CXX_COMPILER NAMES ${COMPILER_PREFIX}-g++)
+find_program(CMAKE_C_COMPILER NAMES ${COMPILER_PREFIX}-gcc-posix)
+find_program(CMAKE_CXX_COMPILER NAMES ${COMPILER_PREFIX}-g++-posix)
 
 SET(CMAKE_FIND_ROOT_PATH /usr/${COMPILER_PREFIX})