bugfix: include config.h before any other headers

For some reason, flex used to prepend several standard headers before
config.h. This prevented some conditional extension in these headers
from being defined.

The explicit prototype for strdup() shouldn't be required anymore as
it should be provided by string.h.
configure: remove hardcoded compiler mode

Specifying the mode explicitly shouldn't be necessary as the
AC_PROG_CC_C99 autoconf macro should determine a compatible mode
itself.
configure: forbid implicitly declared functions
configure: replace deprecated autoconf macro

The new macro depends on autoconf >= 2.60.
Merge pull request #152 from rsyslog/master-th-compilation-cleanup

Several adjustments to the build environment as mentioned on the mailing list