Debug builds of the core (like `ndk-make.sh arm64-v8a`) take about a minute, while
a release bulid (`ndk-make.sh`) takes about 15 minutes.
But as most people seem not to use debug builds regularly because
they are tricky to use, here comes an improvement :) If you gave the wrong argument to `ndk-make.sh`, a warning dialog will now pop up inside the app, telling you what you did wrong and what you should do instead.
`ndk-make.sh` writes its argument into the file `ndkArch`.
`getNdkArch()` in `build.gradle` then reads this file and its content is assigned to
`BuildConfig.NDK_ARCH`. `checkNdkArchitecture()` then checks this.
