[build] Add autogen.sh; delete generated files.

We also add some placeholder files to keep Git happy.

* autogen.sh: New file.
* build/.placeholder: Likewise.
* m4/.placeholder: Likewise.
* INSTALL: Delete file.
* Makefile.in: Likewise.
* acinclude.m4: Likewise.
* aclocal.m4: Likewise.
* build/config.guess: Likewise.
* build/config.sub: Likewise.
* build/depcomp: Likewise.
* build/install-sh: Likewise.
* build/ltmain.sh: Likewise.
* build/missing: Likewise.
* config.h.in: Likewise.
* configure: Likewise.
* examples/Makefile.in: Likewise.
* src/Makefile.in: Likewise.

Author: Thien-Thi Nguyen