message: cmake: Add control over Android build-tools version
  Specify -DANDROID_BUILD_TOOLS_VERSION=<foo> to change from the
  default of 26.0.1.
  Also respect the selected Android platform version.
diff: @@ -64,12 +64,18 @@ if(ANDROID)
  message(FATAL_ERROR "ANDROID_SDK environment variable must be defined for Android build")
  endif()
  - set(BUILD_TOOLS "$ENV{ANDROID_SDK}/build-tools/26.0.1")
  - set(ANDROID_JAR "$ENV{ANDROID_SDK}/platforms/android-23/android.jar")
  + set(ANDROID_BUILD_TOOLS_VERSION "" CACHE STRING "Version of Android build-tools to use instead of the default")
  + if(ANDROID_BUILD_TOOLS_VERSION STREQUAL "")
  + set(ANDROID_BUILD_TOOLS_VERSION "26.0.1")
  + endif()
  + message(STATUS "Using Android build-tools version ${ANDROID_BUILD_TOOLS_VERSION}")
  +
  + set(BUILD_TOOLS "$ENV{ANDROID_SDK}/build-tools/${ANDROID_BUILD_TOOLS_VERSION}")
  set(RT_JAR "$ENV{JAVA_HOME}/jre/lib/rt.jar")
  set(JAVA_BIN "$ENV{JAVA_HOME}/bin")
  set(APK_TARGET_ID "android-23" CACHE STRING "The Target ID to build the APK for, use <android list targets> to choose another one.")
  + set(ANDROID_JAR "$ENV{ANDROID_SDK}/platforms/${APK_TARGET_ID}/android.jar")
  set(KEYSTORE ${CMAKE_CURRENT_BINARY_DIR}/debug.keystore)
  add_custom_command(OUTPUT ${KEYSTORE}
  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
  