debuginfod: Add --disable-libdebuginfod and --enable-libdebuginfod=dummy.

debuginfod: Add --disable-libdebuginfod and --enable-libdebuginfod=dummy.

Make it possible to build just the debuginfod client or to create a
dummy libdebuginfod that doesn't link against libcurl. The dummy library
can be used for bootstrapping. For testing purposes you can also build
debuginfod against the dummy libdebuginfod but then the debuginfod
server will not be able to do delegation.

Signed-off-by: Mark Wielaard <mark@klomp.org>