swiftc(1) requires every -D option to be prefixed with -Xcc, which
is not easy to achieve using autoconf.
Now instead of setting configuration macros like HAVE_LSEEK64 on the
command line using -D, they are defined in config.h, which is generated
by ./configure, ignored by Git, and included by all headers and source
files that use any configuration macro.
The dependency on Makefile has been replaced with a dependency on
Makefile.in and config.h, which helps avoid some redundant full
recompiles after running ./configure, because the configure script
does not overwrit config.h if its content does not change.
