Don't remove version.m2 in "make clean"

[d-torrance]

Otherwise, subsequent calls to make will fail.

It's still removed by make distclean, which makes sense since it's generated by configure and not by make.

This is the current behavior after running make clean:

$ make
make: Entering directory '/home/profzoom/src/macaulay2/M2/M2/BUILD/build'
: "checking for strings that look like undefined configure variables in all *.in files..."
grep: Macaulay2/m2/version.m2: No such file or directory
: creating or removing symbolic link to common staging area, if necessary,
: based on comparison of these directories:
:                      pre_prefix : /home/profzoom/src/macaulay2/M2/M2/BUILD/build/usr-dist/common
: abs_builddir/usr-dist/common : /home/profzoom/src/macaulay2/M2/M2/BUILD/build/usr-dist/common
rm -f srcdir .link-test
echo "../../" >srcdir
chmod: cannot access 'Macaulay2/m2/version.m2': No such file or directory
make: *** [Makefile:110: protect-configs] Error 1
make: Leaving directory '/home/profzoom/src/macaulay2/M2/M2/BUILD/build'