if ${CRC32_LIBRARY} target has no COMPILE_FLAGS yet,
GET_TARGET_PROPERTY returns COMPILE_FLAGS-NOTFOUND, which
doesn't work very well when it's later fed back into COMPILE_FLAGS.

GET_PROPERTY() returns an empty string in this case.

Cherry-picked from 10.4 - 13e8f728Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>