Updated build system.

Updated build system.

Major changes:
- Moved all re2c source files to a separate directory. Top source
  directory should only contain autotools source files and a few other
  files like README.

- Enabled out-of-source builds (and wrote a simple script build.sh
  that makes out-of-source build).

- Improved portable variant of <stdint.h> header (src/c99_stdint.h):
  now it relies only on some few defines in configure-generated
  src/config.h (instead of checking for MSVC version and relying
  on MSVC-defined stuff). Implementation follows C99 standard closely.

- Removed all windows-related build stuff. It was no use keeping it:
  it's been broken for a long time and I can't maintain it.

- Removed all RPM stuff: distro-maintainers use their own hacks anyway.
  Makefile.am is definetely the wrong place to keep such things.
  A separete script and .spec file is a better idea, but again, nobody
  uses it.

- added make target 'bootstrap' to make mainteiners' job easier.

- Merged lessons and examples into one.

- Updated README and doc/index.html.

- Run tests in parallel by default.

Changes concerning particular files:
- configure.ac:
    - removed autoconf version check: developers will need the right
      version anyway (otherwize autoconf will reject to work);
      users should use configure script provided by package
      distribution

    - removed GCC version (3 or above) check: it's pretty ancient
      and I don't know which features are missing anyway

    - removed some useless checks (which resulted in defines in
      src/config,h used by no one)

    - introduced some new checks (used by src/c99_stdint.h)

    - followed the advices of autoupdate

Makefile.am:
    - explicitly prefixed all file names with $(srcdir) or $(builddir)

    - removed windows and RPM related rules and targets

    - added target 'bootstrap'

    - sorted out automake variables