configure.ac: check if flags are needed for c99

configure.ac: check if flags are needed for c99

some older and more obscure compilers (e.g. mipspro on irix) default to the strictest standard they know instead of the most permissive. since libarchive uses c99 features in several places throughout, but does not pass appropriate cflags to parse c99 code, it will fail to compile from the configure script on these systems. this change will have autoconf check if any flags are needed for the c compiler to accept c99 code, and add them to cflags.