* configure.ac: Enable system extensions
Use AC_USE_SYSTEM_EXTENSIONS to build with _GNU_SOURCE (and similar
macros on non-GNU systems), to avoid an implicit declaration of the
strptime function. This improves compatibility with future compilers
which do not support implicit function declarations.
* configure.ac: gettimeofday lives in <sys/time.h>
The gettimeofday function is declared in <sys/time.h>, not <time.h>,
according to POSIX and actual systems. This avoids a configure
probe failure with compilers which do not support implicit function
declarations.
