build: avoid bootstrap failure when $GZIP is set

build: avoid bootstrap failure when $GZIP is set

Running "GZIP=-9 ./bootstrap" would fail right away, because the
tool-version-checking code would treat the upper-cased program name
as an environment variable name and if that has a value use the
result as the application name.  That works fine for automake,
autoconf, etc. but not for gzip.
* bootstrap (check_versions): Do not treat $GZIP as a program name.
If defined at all, it is supposed to list gzip options.
Reported by Alan Curry in http://debbugs.gnu.org/8609