Use check_lib2 to test the header together with the function. This is
necessary, because '-DOPJ_STATIC' changes what the included header does.

Also add '-DOPJ_STATIC' to CPPFLAGS, so that it isn't necessary to
hardcode this in libavcodec/libopenjpeg{dec,enc}.c.

Finally, check for non-static openjpeg 2.1, too.
Reviewed-by: Michael Bradshaw <mjbshaw@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>