Update the configure.ac for autoconf >= 2.71 compatibility
This mostly removes stuff that's either deprecated, obsolete or not used at all:
- AC_PROG_CC_C99 is deprecated, just use AC_PROG_CC as we require C11 anyway
- AC_HEADER_TIME is deprecated, both <sys/time.h> and <time.h> can be included at the same time, and we don't use the macros that AC_HEADER_TIME defines anywhere
- AC_HEADER_STDC checks for ISO C90 and we require at least C11
- Replace AC_TRY_([]) with AC__IFELSE([AC_LANG_PROGRAM()])
Edited by Ondřej Surý