Add support -f --disable-tests.

* Makefile.am: Use BUILD_TESTS for tests.
* configure.ac (BUILD_TESTS): Add support of --disable-tests.

configure.ac code is from libgpg-error.

• Reported-by: Jernej Jakob <jernej.jakob@gmail.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>