Compiles only if ndk-build is in the $PATH.

If not, it warns the user.
Looks for correct ndk-build command.

If the build-native.sh script points to a different ndk-build
executable, it uses it.

If not, compile.sh looks for a build-native.sh in the $PATH and, if
present, correct the possible erroneous path in build-native (just to
avoid silly edits in that file), and compiles with it.