Enable caching values generated by the Android build system

Enable caching values generated by the Android build system

* build-aux/ndk-build-helper-1.mk:

* build-aux/ndk-build-helper-2.mk:

* build-aux/ndk-build-helper.mk: Do not generate file names with
redundant directory separators, to port to systems where the
semantics of this are undefined.

* configure.ac: Provide a cache file to the recursive invocation
of configure if one is specified for the initial.

* cross/ndk-build/Makefile.in (my-dir): Better conform to the
original ndk-build by generating directory names with no
trailing separator.

* m4/ndk-build.m4 (ndk_resolve_import_module, ndk_SEARCH_MODULE):
Cache the names of Android.mk files and the commands produced
from them.

(cherry picked from commit cab91300d0df2250bc80c0cba15691fe5f8172fb)