Improve reliability of Android build system

Improve reliability of Android build system

* .gitignore: Add new files.
* INSTALL.android: New file.
* Makefile.in (clean_dirs): Clean xcompile as well.
* admin/merge-gnulib (avoided_flags): Import gnulib into Android
directory as well.
* doc/emacs/android.texi (Android):
* doc/emacs/emacs.texi (Top): New node `Android'.
* java/org/gnu/emacs/EmacsThread.java (run): Use right
executable name.
* lib/Makefile.in (ANDROID_CFLAGS): Use better way to refer to
/src.
(vpath): Delete ugly block of vpath statements.
(mostlyclean): Remove Makefile.android.
* lib/fpending.c (__fpending):
* lib/open.c:
* lib/unistd.c (_GL_UNISTD_INLINE): Revert changes to gnulib in
lib/.
* src/android.h:
* src/androidterm.c: Fix build.
* xcompile/Makefile.in (LIB_SRCDIR):
(LIBSRC_BINARIES, src/verbose.mk):
(PRE_BUILD_DEPS, PHONY): Use gnulib in xcompile/lib/ as opposed
to lib/.
* xcompile/README: Adjust README.