opal/configury: allow param usage multiple times

Turns out the OPAL_FLAGS_UNIQ function was chopping
off multiple instances of --param when set in the
CFLAGS.  This can happen when -mnative or other machine
target is specified as part of CFLAGS.

Thanks to QuesarVII for reporting this and supplying a patch.

Tested using -mnative to see that gcc was being passed the right
set of --param options multiple times.

@rhc54 would you mind reviewing this?  

Fixes #324