README.md: note about toolchain/android-14/bin in PATH and wrt existing Chicken in it
README.md: Add Chicken as a prerequisite

A working Chicken is needed. Since we touch eval.scm, the build system
detects it was modified and tries to compile it to eval.c.  We need an
installed chicken for that.
Makefile: make chicken version configurable
Makefile: NDK_PATH can be configured via command line