package/mbuffer: do not force c99 mode

package/mbuffer: do not force c99 mode

Do not force c99 mode to be able to remove both patches and because it
does not seem to be motivated. Indeed, even if the second patch was
sucessfully upstreamed. The first one can't be upstreamed as it raises
the following build falure on Solaris 11 (due to mixing c99 and
_POSIX_C_SOURCE):

 #if defined(STDC_C99) && (defined(_XOPEN_OR_POSIX) && !defined(XPG6))
 #error "Compiler or options invalid for pre-UNIX 03 X/Open applications \
 and pre-2001 POSIX applications"

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>