configure.ac: add '--disable-tests' option

For toolchains without c++ support add option to disable
tests, because this is the only place c++ is needed (to
test the c++ binding).

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
- renamed "tests" into "enable_tests"
- used yes/no instead of true/false
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>