Add configure option to stop examples being built, --disable-examples.

- Added new configure option --disable-examples which can be used to
  stop the example programs being built. This can be useful in some
  situations, such as when cross compiling static libraries.

- Examples are still built by default, --disable-examples must be passed
  explicitly to stop them being built.

Signed-off-by: Ralph Giles <giles@thaumas.net>