fix(configure.ac): bourne sh compatibility

Co-authored-by: Koichi Murase <myoga.murase@gmail.com>

Author: scop

configure.ac

@@ -8,7 +8,7 @@ AC_PROG_LN_S
 AC_PROG_MKDIR_P
 AC_PROG_SED
 AC_ARG_WITH([pytest],[  --with-pytest=executable],[PYTEST="$withval"])
-if test ! "$PYTEST"; then
+if test "x$PYTEST" = x; then
     AC_CHECK_PROGS([PYTEST],[pytest pytest-3],[pytest])
 fi
 AC_CONFIG_FILES([