Don't automatically regenerate uconfig.h as part of the microperl build.

Don't automatically regenerate uconfig.h as part of the microperl build.

This avoids limiting microperl to platforms with a Bourne shell. It also
avoids automated build tools modifying distribution files as part of the
default build targets.

Now that we have a regression test to ensure that uconfig.h is up to date,
the version as shipped will be consistent with uconfig.sh. README.micro
already instructs people to run `make -f Makefile.micro regen_uconfig` if
they edit uconfig.sh, so no change needed there.