Fix stack overflow that can occur in libevhtp

libevhtp allocates a stack based on data length
when C99 is detected at compile time. There are
no checks to verify that the stack is big enough
which can cause a stack overflow.

Adding EVHTP_HAS_C99=false at compile time changes
this behavior by allocate to a buffer which has
proper checks in place.

More information about this bug can be found at:
https://github.com/criticalstack/libevhtp/issues/118
https://github.com/haiwen/seafile/issues/1928

MFH: 2019Q3