restore CFLAGS check for conflict and fix recursive CFLAGS issue

cc-option uses more make-syntax to replace the shell "if/else".

Issue with recursive += is fixed by doing the first assignment
simply-expanded, as explained in
http://www.gnu.org/software/make/manual/html_node/Appending.html

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
