Allow c99 code to compile

Add the appropriate compiler flags to accept c99 code. This will help to
minimize differences with upstream, and aid porting changes. One change was
necessary in zvol.c because the DEFINE_IDA() macro does not work with the new
compiler flags.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #5756