tests: fix Makefile so it is portable to gmake 3.x

In commit 0355d6d047884f5b we used a Makefile shell assignment syntax
which is compatible with pmake and gmake 4.x, but which breaks on
gmake 3.x.  So only use the pmake-compatible construction if we are
not using GNU make.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
1 file changed