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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.)