mroonga_libgroonga_support_lz4 is set to ON even when lz4 is not available, CREATE behavior changes

The variable mroonga_libgroonga_support_lz4 is documented as OFF by default. In fact, on older versions, e.g. on 10.6, it appears to be system/build-dependent: on builds where lz4 is available it is set to ON by default, otherwise OFF. For example on the latest 10.6 build from buildbot, on Debian-10 it is ON and on SLES-12 it is OFF. When a table with lz4 compression is attempted to be created, if lz4 support is OFF, a warning is issued ("Warning 16506 The column flag 'COMPRESS_LZ4' is unsupported. It is ignored") and the compression is ignored.

However on 10.7 with compression providers, mroonga_libgroonga_support_lz4 is always ON, even on the same SLES-12 which doesn't have lz4 at all. And on Debian-10 it is ON regardless whether the provider is loaded or not. And then, when a table with lz4 compression is attempted to be created, an error ER_PROVIDER_NOT_LOADED occurs instead of the warning described above.

The test case is just running something like

create table test.t1 (a int, b text COMMENT 'FLAGS "COLUMN_SCALAR|COMPRESS_LZ4"') engine = mroonga charset = utf8;

with a specific combination of the variable and the providers.