Update copyright years.

Update copyright years.
Delete config.log before doing anything else.
Write additional diagnostics to config.log in case of failures to
compile test programs, including the failed test program itself.
Add a test for support of -mtune=pentium4 switch to GCC; if it is
supported, set up MCPU_FLAG variable on the various Makefiles to
use that switch during compilations.  (This avoids GCC warning
about -mcpu being deprecated.)