Travis CI: add build configuration for Android NDK with cmake.
diff --git a/.travis.yml b/.travis.yml
index 7a97971..8e4edb2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,6 +25,9 @@
   - gcc
 
 matrix:
+  include:
+    # Additional build using Android NDK
+    - env: BUILD_NDK=ON
   exclude:
     # Skip GCC builds on Mac OS X.
     - os: osx
@@ -45,6 +48,12 @@
 before_install:
   # Install cmake & ninja on Mac OS X.
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update && brew install cmake ninja; fi
+  - if [[ "$BUILD_NDK" == "ON" ]]; then
+      git clone --depth=1 https://github.com/urho3d/android-ndk.git $HOME/android-ndk;
+      export ANDROID_NDK=$HOME/android-ndk;
+      git clone --depth=1 https://github.com/taka-no-me/android-cmake.git $HOME/android-cmake;
+      export TOOLCHAIN_PATH=$HOME/android-cmake/android.toolchain.cmake;
+    fi
 
 before_script:
   - git clone https://github.com/KhronosGroup/SPIRV-Headers.git external/spirv-headers
@@ -52,9 +61,18 @@
 
 script:
   - mkdir build && cd build
-  - cmake -GNinja -DCMAKE_BUILD_TYPE=${BUILD_TYPE} ..
+  - if [[ "$BUILD_NDK" == "ON" ]]; then
+      cmake -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_PATH}
+            -DANDROID_NATIVE_API_LEVEL=android-9
+            -DCMAKE_BUILD_TYPE=Release
+            -DANDROID_ABI="armeabi-v7a with NEON"
+            -DSPIRV_SKIP_TESTS=ON
+            -GNinja ..;
+    else
+      cmake -GNinja -DCMAKE_BUILD_TYPE=${BUILD_TYPE} ..;
+    fi
   - ninja
-  - ctest -j`nproc` --output_on_failure
+  - if [[ "$BUILD_NDK" != "ON" ]]; then ctest -j`nproc` --output_on_failure; fi
 
 
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